Prevent no-op statements in Metal code generator.

These are harmless, but they can cause the Metal compiler to emit a
warning, and these warnings are making some logs messy for Flutter.

Change-Id: I21b7a3c9ae661c55bbbe8bc13d097966180e977d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/496377
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
6 files changed