Evenly space gradient stage.

Evenly space gradient stage.

This seems like an experiment at this point because I don't know how to do
this kind of thing on arm.

Numbers from Skylake...
Before:
./out/Release/nanobench --config srgb \
--match gradient_linear_clamp_3color gradient_linear_clamp_hicolor -q                                                                        19:48:13
Timer overhead: 36.7ns
! -> high variance, ? -> moderate variance
    micros      bench
    439.92 ?    gradient_linear_clamp_3color    srgb
   2697.60      gradient_linear_clamp_hicolor   srgb
    437.28      gradient_linear_clamp_3color_4f srgb
   2700.50      gradient_linear_clamp_hicolor_4f        srgb

After:
   micros      bench
    382.35      gradient_linear_clamp_3color    srgb
    593.49      gradient_linear_clamp_hicolor   srgb
    382.36      gradient_linear_clamp_3color_4f srgb
    565.60      gradient_linear_clamp_hicolor_4f        srgb

Numbers on my Mac Trashcan are about even; there is no
speedup or slowdown between master and this change.

Change-Id: I04402452e23c0888512362fd1d6d5436cea61719
Reviewed-on: https://skia-review.googlesource.com/15960
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>