This CL adds the Bitcast expression into the IR.
Bug: tint:1718
Change-Id: Ic48bd54485e9b380c94f599e683c2fbba7505787
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116041
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>