clean up SkBlitRow_opts

SSE2 and NEON are common baseline instruction sets now,
so there's no need to runtime detect support for these routines.

I simplified the SSE and portable implementations while moving them.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I34e96851735c8d7ad90198f3ac4bf86ff508f17c
Reviewed-on: https://skia-review.googlesource.com/c/170220
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
10 files changed