add pack instruction

This instruction can lower to some useful SSE/NEON
instructions, and even if not, is a handy way to
express the frequent paring of << and |.

I32_SWAR:  2.3 -> 1.9
I32:       2.6 -> 2.4
F32:       5.1 -> 4.7

Change-Id: Ia169ad40f0aaef32417e05d9bf91c2d2542e7b5f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218238
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed