Currently polyfilled for all backends.
HLSL should be able to map this to 'firstbitlow', but there might need
to be some special case handling for 0 (undocumented behavior). For now
just polyfill.
CTS tests: https://github.com/gpuweb/cts/pull/1003
Bug: tint:1367
Bug: tint:1449
Change-Id: I8125b32687196678906e5a9d056b4f2efd885073
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/81502
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Ben Clayton