update memset16/32 inlining heuristics

update memset16/32 inlining heuristics

I spent some time looking at perf.skia.org and it looks like we can do better.

It is weird, weird, weird that on x86, we see three completely different behaviors:
  - x86 Android: inlining better for small N, custom better for large N;
  - Windows:     inlining better for large N, custom better for small N;
  - other x86:   inlining generally better

BUG=skia:4316,chromium:516426

(Temporary, plan to revert.)
TBR=reed@google.com

Review URL: https://codereview.chromium.org/1357193002