simplify nibble_to_vpshufb

Just realized the mapping from our bytes() control nibble
to vpshufb's control byte is even easier than we thought.

Change-Id: I86fec8d3ca091ecc3fbbd59fe51ac128f0cef2fa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/224316
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed