Zero tail stack buffers.

Zero tail stack buffers.

MSAN and the no-SIMD bots have noticed the top lanes of the tail vectors are  not initialized.

As they were written it was faster to leave them unintialized, but as re-written here it's equal speed and now safe.

CQ_INCLUDE_TRYBOTS=master.client.skia:Perf-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Debug-MSAN-Trybot,Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3790

Change-Id: Icd41ba14ae6baf9947eb361a366f1ce19ad8aa67
Reviewed-on: https://skia-review.googlesource.com/3790
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>