Optimize SkAlphaMulQ_SSE2

These two mask clear are useless, because _mm_srli_epi16 fills high byte
of each word with 0.

BUG=skia:

Review URL: https://codereview.chromium.org/724333003
add gm for 2-rects and blur

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/728863002
Don't pass null to memcmp.

This will allow us to add nonnull-attribute to the UBSAN bot.

We are in fact hitting a case where one of the arguments is null and the other
not, which seems dicey.  I think the scenario is comparing the empty pathref
with another path ref that's just been COWed, without any verbs or points yet.

BUG=skia:

Review URL: https://codereview.chromium.org/732643002