spirv-reader: Be slightly more defensive

Return early if error occurs converting decorations for pipeline
outputs.

Also add slightly better comments.

Bug: tint:508
Change-Id: I5a80f8bbc3bc9196ed70577be3884fe4ba4bba0a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54461
Auto-Submit: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: David Neto <dneto@google.com>

src/reader/spirv/function.cc

@@ -994,7 +994,9 @@ bool FunctionEmitter::EmitEntryPointAsWrapper() {
               source, builder_.Symbols().Register(ep_info_->inner_name)),
           ast::ExpressionList{})));
 
+  // Pipeline outputs are mapped to the return value.
   if (ep_info_->outputs.empty()) {
+    // There is nothing to return.
     return_type = ty_.Void()->Build(builder_);
   } else {
     // Pipeline outputs are converted to a structure that is written
@@ -1018,6 +1020,10 @@ bool FunctionEmitter::EmitEntryPointAsWrapper() {
       if (!parser_impl_.ConvertDecorationsForVariable(
               var_id, &forced_store_type, &out_decos)) {
         // This occurs, and is not an error, for the PointSize builtin.
+        if (!success()) {
+          // But exit early if an error was logged.
+          return false;
+        }
         continue;
       }
 

src/reader/spirv/function.h

@@ -427,6 +427,8 @@ class FunctionEmitter {
   ParserImpl* parser() { return &parser_impl_; }
 
   /// Emits the entry point as a wrapper around its implementation function.
+  /// Pipeline inputs become formal parameters, and pipeline outputs become
+  /// return values.
   /// @returns false if emission failed.
   bool EmitEntryPointAsWrapper();