trim some fat from SSE2 fixed point alpha code

  - extract alpha from a pixel: 5 1-cycle ops to 4 1-cycle ops
  - load alphas: drop 4 unnecessary ops

Should be no pixel diffs.

BUG=skia:
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1447273004
2 files changed