Implement ignore() intrinsic

Fixed: tint:871
Change-Id: I01e275686094611d67a54735593320bce16705c3
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53681
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/sem/intrinsic_type.cc

@@ -127,6 +127,9 @@ IntrinsicType ParseIntrinsicType(const std::string& name) {
   if (name == "fwidthFine") {
     return IntrinsicType::kFwidthFine;
   }
+  if (name == "ignore") {
+    return IntrinsicType::kIgnore;
+  }
   if (name == "inverseSqrt") {
     return IntrinsicType::kInverseSqrt;
   }
@@ -353,6 +356,8 @@ const char* str(IntrinsicType i) {
       return "fwidthCoarse";
     case IntrinsicType::kFwidthFine:
       return "fwidthFine";
+    case IntrinsicType::kIgnore:
+      return "ignore";
     case IntrinsicType::kInverseSqrt:
       return "inverseSqrt";
     case IntrinsicType::kIsFinite:

src/sem/intrinsic_type.h

@@ -66,6 +66,7 @@ enum class IntrinsicType {
   kFwidth,
   kFwidthCoarse,
   kFwidthFine,
+  kIgnore,
   kInverseSqrt,
   kIsFinite,
   kIsInf,