remove opaque/alpha distinction in SSSE3 code

Just always use the alpha path.  Some microbenchmarks
get a little slower, but no .skps do, even ones that are
using S32_opaque_D32_filter_DX_SSSE3 heavily.

Change-Id: I4aa5f52165e504b4fcd00fff28b0a649ad4f0716
Reviewed-on: https://skia-review.googlesource.com/c/170763
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed