spirv-reader: explicitly error out on extended arithmetic

The feature is not yet finished in WGSL.

Fixed: tint:804
Change-Id: I5268126f68316e57ee60d099f2000223703845df
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/55500
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: David Neto <dneto@google.com>

src/reader/spirv/function.cc

@@ -3540,6 +3540,14 @@ bool FunctionEmitter::EmitStatement(const spvtools::opt::Instruction& inst) {
       }
       break;
 
+    case SpvOpIAddCarry:
+    case SpvOpISubBorrow:
+    case SpvOpUMulExtended:
+    case SpvOpSMulExtended:
+      return Fail() << "extended arithmetic is not finalized for WGSL: "
+                       "https://github.com/gpuweb/gpuweb/issues/1565: "
+                    << inst.PrettyPrint();
+
     default:
       break;
   }