Handle spirv-opt struct deduplication for constants

When retrieving a constant struct value from spirv-opt, it may have
deduplicated the struct used as the type. This means the type we get in
the constant won't match our expected type. Check that the types match,
and if not rebuild the constant with the correct type.

This is true from both constants and constant null values, both need to
be handled.

Fixed: 504511706
Change-Id: I2c9bc1907a635c2091f4c2ced6f3a4caf2ae0287
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/307935
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>