tint/transform: fix PromoteInitializersToLet for constant expressions
Fix more edge cases uncovered with tint:1781
Fixed: tint:1781
Change-Id: I58d120185f47c10bc9fe55dd95a198d496c4ec94
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113024
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>