Refactor HInferRepresentation into an HPhase and use the phase zone.

Rename HInferRepresentation to HInferRepresentationPhase,
following naming scheme suggested by danno@chromium.org in
https://codereview.chromium.org/17458002

The HInferRepresentationPhase now uses the phase zone for all its
allocations.

Depends on https://codereview.chromium.org/17587008

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/18054002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15373 ce2b1a6d-e550-0410-aec6-3dcde31c8c00