[turbofan] remove hint aliasing

[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=

Committed: https://crrev.com/232b09825c2b1b7c2356f483a1b50c305863ad5f
Cr-Commit-Position: refs/heads/master@{#28075}

[Sven Panne, 2015-04-27 12:21:04]

svenpanne@chromium.org changed reviewers:
+ svenpanne@chromium.org

[Sven Panne, 2015-04-27 12:21:05]

LGTM (rubber-stamped :-} )

[dcarney, 2015-04-27 12:22:54]

The CQ bit was checked by dcarney@chromium.org

[commit-bot: I haz the power, 2015-04-27 12:23:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1103533002/200001

[commit-bot: I haz the power, 2015-04-27 12:46:52]

Committed patchset #11 (id:200001)

[commit-bot: I haz the power, 2015-04-27 12:47:03]

Patchset 11 (id:??) landed as
https://crrev.com/232b09825c2b1b7c2356f483a1b50c305863ad5f
Cr-Commit-Position: refs/heads/master@{#28075}