If the object and index are both constant expressions, resolve the
index as a constant.
Note: Expectations have been updated for indexing on 'let's. Once
'const' is introduced, 'let' will no longer be treated as a constant
expression, and these expectations will be reverted.
Bug: tint:1580
Change-Id: I42793d36c1a5f82890ccaa5dbbb71b4346493bef
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94329
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
c1cb9dc1a5