baby steps into 16-bit ops

I figure the easiest way to expose 16-bit operations
is to expose 16x2 pair operations... this means we
can continue to always work with the same size vector.

Switching from 32-bit multiplies to 16-bit multiplies
is going to deliver the most oomph... they cost roughly
half what 32-bit multiplies do on x86.

Speed now:

   I32_SWAR: 0.27 ns/px
   I32:      0.43 ns/px
   F32:      0.76 ns/px
   RP:       0.8  ns/px
   Opts:     0.2  ns/px

Change-Id: I8350c71722a9bde714ba18f97b8687fe35cc749f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220709
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
5 files changed