Fix bugs in SkBmpCodec

The decode should not depend on the requested alpha type.

These were exposed by:
https://codereview.chromium.org/1641273003

This should cause the number of untriaged images in Gold
to go to zero.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1663303002

Review URL: https://codereview.chromium.org/1663303002
diff --git a/src/codec/SkMaskSwizzler.cpp b/src/codec/SkMaskSwizzler.cpp
index 1b77a85..7630a7b 100644
--- a/src/codec/SkMaskSwizzler.cpp
+++ b/src/codec/SkMaskSwizzler.cpp
@@ -235,18 +235,19 @@
         case 16:
             switch (dstInfo.colorType()) {
                 case kN32_SkColorType:
-                    switch (dstInfo.alphaType()) {
-                        case kUnpremul_SkAlphaType:
-                            proc = &swizzle_mask16_to_n32_unpremul;
-                            break;
-                        case kPremul_SkAlphaType:
-                            proc = &swizzle_mask16_to_n32_premul;
-                            break;
-                        case kOpaque_SkAlphaType:
-                            proc = &swizzle_mask16_to_n32_opaque;
-                            break;
-                        default:
-                            break;
+                    if (kOpaque_SkAlphaType == srcInfo.alphaType()) {
+                        proc = &swizzle_mask16_to_n32_opaque;
+                    } else {
+                        switch (dstInfo.alphaType()) {
+                            case kUnpremul_SkAlphaType:
+                                proc = &swizzle_mask16_to_n32_unpremul;
+                                break;
+                            case kPremul_SkAlphaType:
+                                proc = &swizzle_mask16_to_n32_premul;
+                                break;
+                            default:
+                                break;
+                        }
                     }
                     break;
                 case kRGB_565_SkColorType:
@@ -259,18 +260,19 @@
         case 24:
             switch (dstInfo.colorType()) {
                 case kN32_SkColorType:
-                    switch (dstInfo.alphaType()) {
-                        case kUnpremul_SkAlphaType:
-                            proc = &swizzle_mask24_to_n32_unpremul;
-                            break;
-                        case kPremul_SkAlphaType:
-                            proc = &swizzle_mask24_to_n32_premul;
-                            break;
-                        case kOpaque_SkAlphaType:
-                            proc = &swizzle_mask24_to_n32_opaque;
-                            break;
-                        default:
-                            break;
+                    if (kOpaque_SkAlphaType == srcInfo.alphaType()) {
+                        proc = &swizzle_mask24_to_n32_opaque;
+                    } else {
+                        switch (dstInfo.alphaType()) {
+                            case kUnpremul_SkAlphaType:
+                                proc = &swizzle_mask24_to_n32_unpremul;
+                                break;
+                            case kPremul_SkAlphaType:
+                                proc = &swizzle_mask24_to_n32_premul;
+                                break;
+                            default:
+                                break;
+                        }
                     }
                     break;
                 case kRGB_565_SkColorType:
@@ -283,18 +285,19 @@
         case 32:
             switch (dstInfo.colorType()) {
                 case kN32_SkColorType:
-                    switch (dstInfo.alphaType()) {
-                        case kUnpremul_SkAlphaType:
-                            proc = &swizzle_mask32_to_n32_unpremul;
-                            break;
-                        case kPremul_SkAlphaType:
-                            proc = &swizzle_mask32_to_n32_premul;
-                            break;
-                        case kOpaque_SkAlphaType:
-                            proc = &swizzle_mask32_to_n32_opaque;
-                            break;
-                        default:
-                            break;
+                    if (kOpaque_SkAlphaType == srcInfo.alphaType()) {
+                        proc = &swizzle_mask32_to_n32_opaque;
+                    } else {
+                        switch (dstInfo.alphaType()) {
+                            case kUnpremul_SkAlphaType:
+                                proc = &swizzle_mask32_to_n32_unpremul;
+                                break;
+                            case kPremul_SkAlphaType:
+                                proc = &swizzle_mask32_to_n32_premul;
+                                break;
+                            default:
+                                break;
+                        }
                     }
                     break;
                 case kRGB_565_SkColorType: