add extract instruction

Kind of the flip side of pack.

Made slightly awkward by instructions having only one immediate...
calling _BitScanForward / __builtin_ctz() at runtime seems to work
fine, but it really could have been done at compile time.

Change-Id: Ic83fe8e0a1603fb9189598dcc26c842cc797bf45
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218241
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed