swap_src_dst -> move_src_dst

swap_src_dst -> move_src_dst

We thought it'd be handy to have swap_src_dst so that it could be used
either to move dst into src or src into dst.  Turns out, we already have
a stage that moves dst into src (called "dst", the dst blend mode), so
there's really no reason to have swap_src_dst over the strictly more
efficient move_src_dst.

swap_src_dst is typically 12 register moves, where move_src_dst is 4.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4461

Change-Id: Ib453775f854e313f823851978eaadc3995872312
Reviewed-on: https://skia-review.googlesource.com/4461
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>