Clean up Metal bitcast implementation.

I didn't notice the intrinsic map the first time around; this is better
than a bunch of string comparisons in a row.

This doesn't change the results, just changes how we get there.

Change-Id: Iab250b71b6bccc4ff0e0cce6f23658985d48ac59
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343097
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed