This adds a new hint type for live ranges that allows the control
flow aware allocation to hint the register that should be used on
reload. Avoid unnecessary register to register moves and helps
with code size.

Change-Id: I01e870514446eab3bffb89b2e00644bd215b81d5
Reviewed-on: https://chromium-review.googlesource.com/c/1460944Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59698}