Fix SkColorSpaceXform F16 behavior

Fix SkColorSpaceXform F16 behavior

After changing the matrix representation, we are no longer guaranteed
that the "a" in "rgba" is zeroed in store_f16_1() opaque.

This fixed Gold:
https://gold.skia.org/search?blame=26a0543579cf7473de2099ce0d056ac8aba83811&head=true&include=false&limit=50&neg=false&pos=false&query=source_type%3DcolorImage&unt=true

Also tacked on an optimization.  No need to load alpha when the image
is opaque.

BUG=skia:

Change-Id: I28d80c486b6644eee4a38a7a70f94eb51474e4b0
Reviewed-on: https://skia-review.googlesource.com/6738
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>