Implement bitCount intrinsic on SPIR-V and Metal.

This intrinsic was previously lacking a unit test, and wasn't actually
implemented in Metal or SPIR-V. Fortunately it's trivial to add.

Change-Id: I68bbdc58376b579c7f3f0ae5f49323b389c2b8c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/346263
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed