[crankshaft] Cleanup representation calculation for Phis.

[crankshaft] Cleanup representation calculation for Phis.

This replaces the counters for use representations with
simple tracking of most-general representation seen so far.

R=bmeurer@chromium.org

Committed: https://crrev.com/e0241e42ab9a9fb0a7c76fcb67b504e1916ce90b
Cr-Commit-Position: refs/heads/master@{#30506}

[Jarin, 2015-09-01 10:47:06]

Could you take a look, please?

[Benedikt Meurer, 2015-09-01 10:51:14]

LGTM. Looks better indeed. But we should be careful not to fix too many issues
with Crankshaft :-)

[Jarin, 2015-09-01 10:51:47]

The CQ bit was checked by jarin@chromium.org

[commit-bot: I haz the power, 2015-09-01 10:51:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1315193010/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1315193010/20001

[commit-bot: I haz the power, 2015-09-01 11:16:02]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-01 11:16:14]

Patchset 2 (id:??) landed as
https://crrev.com/e0241e42ab9a9fb0a7c76fcb67b504e1916ce90b
Cr-Commit-Position: refs/heads/master@{#30506}