Randomize the order that registers appear in the free list.  Only
randomize fully "equivalent" registers to ensure no extra spills.

This adds the -randomize-regalloc option.

This is a continuation of https://codereview.chromium.org/456033003/ which Matt owns.

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/807293003