make some SkVM benches uneven

N=15 and N=63 make for nice even looking profiles
on ARM and x86 respectively, with N=15 running 3
body loops and 3 tail loops on ARM, N=63 running
7 body loops and 7 tail loops on x86.

Change-Id: Ie7616bd99c949328bbb7d7048fc6f468ff1e3ad2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227220
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed