Reland "Revert "Make FP optimizations helpers use SkAlphaType not GrColorType""

This reverts commit 997b37fdb986b45ff9941319b0111b188e749cfc.

Reason for revert: May be blocking Chrome roll

Original change's description:
> Revert "Revert "Make FP optimizations helpers use SkAlphaType not GrColorType""
> 
> This reverts commit 078e8faa26d8b1f33a92e57f587be011150d1776.
> 
> Change-Id: I67b2970584db5a1afbf9928b77c5444947ef71a3
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255897
> Reviewed-by: Brian Salomon <bsalomon@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

TBR=egdaniel@google.com,bsalomon@google.com

Change-Id: I0d2754ac7f4672f0758e4d00cd9e06998637846a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/256196
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/effects/generated/GrSimpleTextureEffect.cpp b/src/gpu/effects/generated/GrSimpleTextureEffect.cpp
index fd8362a..279ea5d 100644
--- a/src/gpu/effects/generated/GrSimpleTextureEffect.cpp
+++ b/src/gpu/effects/generated/GrSimpleTextureEffect.cpp
@@ -85,8 +85,7 @@
                                              : GrSamplerState::Filter::kNearest);
 
     const SkMatrix& matrix = GrTest::TestMatrix(testData->fRandom);
-    auto alphaType = static_cast<SkAlphaType>(
-            testData->fRandom->nextRangeU(kUnknown_SkAlphaType + 1, kLastEnum_SkAlphaType));
-    return GrSimpleTextureEffect::Make(testData->textureProxy(texIdx), alphaType, matrix, params);
+    return GrSimpleTextureEffect::Make(testData->textureProxy(texIdx),
+                                       testData->textureProxyColorType(texIdx), matrix, params);
 }
 #endif