Add support for input attachments in SkSL spirv

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5115

Change-Id: I3e03a465a10c9aff62491d0f6e71105d1b650dab
Reviewed-on: https://skia-review.googlesource.com/5115
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/src/sksl/SkSLSPIRVCodeGenerator.h b/src/sksl/SkSLSPIRVCodeGenerator.h
index 84c582e..b791945 100644
--- a/src/sksl/SkSLSPIRVCodeGenerator.h
+++ b/src/sksl/SkSLSPIRVCodeGenerator.h
@@ -51,7 +51,7 @@
     class LValue {
     public:
         virtual ~LValue() {}
-        
+
         // returns a pointer to the lvalue, if possible. If the lvalue cannot be directly referenced
         // by a pointer (e.g. vector swizzles), returns 0.
         virtual SpvId getPointer() = 0;
@@ -84,7 +84,8 @@
         kAtan_SpecialIntrinsic,
         kTexture_SpecialIntrinsic,
         kTexture2D_SpecialIntrinsic,
-        kTextureProj_SpecialIntrinsic
+        kTextureProj_SpecialIntrinsic,
+        kSubpassLoad_SpecialIntrinsic,
     };
 
     void setupIntrinsics();