SkSplicer: begin on sse2/sse4.1 support

SkSplicer: begin on sse2/sse4.1 support

This lets us target older machines with SkSplicer.
SSE2 and SSE4.1 are the sweet spots if we're going to pick two more.

Nothing too interesting here except maybe the f16<->f32 code.

I rearranged a little to keep things consistent across platforms.

Next CL will get this into _generated.h and use it when appropriate.

Change-Id: Ibbdc61ea7a45d22b4f4058b01f75161ea74a7726
Reviewed-on: https://skia-review.googlesource.com/8193
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>