[turbofan] make register hinting explicit

[turbofan] make register hinting explicit

- instead of committing operands early to resolve hints, hold the hint register data on the UsePosition
- allow hints to be rolled back efficiently as needed by GreedyAllocator
- some small drive by fixes

BUG=

Review URL: https://codereview.chromium.org/1103533002

Cr-Commit-Position: refs/heads/master@{#28075}