Add SSE, SSE4.1, and NEON specializations.
The if_then_else() unit tests in SkVxTest.cpp should cover this.
I had to give up on my dream of not using Skia headers for now. There's
really no good way of knowing whether we've got SSE4.1 support in MSVC
except when we explicitly define SK_CPU_SSE_LEVEL=SK_CPU_SSE_LEVEL_SSE41.
This refactor to use SK_CPU_SSE_LEVEL let MSVC point out a slight
ordering problem that would cause an infinite loop calling any of
the specializions like sqrt(float2). I believe moving them after
the float4 specializations will fix that.
Change-Id: I83639f378a182716d1b37e92b6d725472698f874
Reviewed-on: https://skia-review.googlesource.com/c/195920
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
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