Fixup shorten64-to-32 warning

Change-Id: I2995d11693f6f49f67b7d3a11dde8e3fdc8dc549
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18401
Reviewed-by: David Neto <dneto@google.com>

src/reader/spirv/parser_impl.cc

diff --git a/src/reader/spirv/parser_impl.cc b/src/reader/spirv/parser_impl.cc
index 27ec8c6..571bd57 100644
--- a/src/reader/spirv/parser_impl.cc
+++ b/src/reader/spirv/parser_impl.cc
@@ -464,7 +464,8 @@
   // Compute members
   std::vector<std::unique_ptr<ast::StructMember>> ast_members;
   const auto members = struct_ty->element_types();
-  for (size_t member_index = 0; member_index < members.size(); ++member_index) {
+  for (uint32_t member_index = 0; member_index < members.size();
+       ++member_index) {
     auto* ast_member_ty = ConvertType(type_mgr_->GetId(members[member_index]));
     if (ast_member_ty == nullptr) {
       // Already emitted diagnostics.