funnel f16 through standard load/store/gather

funnel f16 through standard load/store/gather

This is a precursor to using mask load, mask store, and gather instructions for f16.  This is a slight performance win too, through slightly simpler code generation.  Having done this, it now makes sense to give a name to f16->f32 conversion, from_f16().

Finally, while we're at this, also send store_f32 through store(), so that now all formats use load, gather, and store uniformly.

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: I403f16f712936e2bcf3294e72c863cb6c6fbcf0c
Reviewed-on: https://skia-review.googlesource.com/5731
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>