PromoteInitializersToLet was not handling sem::Materialize nodes.
This can happen for const arrays when they are dynamically indexed.
Fixed: tint:1653
Change-Id: I3d67d8139e481c89b31a3a30c7ef44384b7545ba
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/99500
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Ben Clayton