Refactor MacroAssembler::Allocate a little bit for X64

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/18660002

Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15532 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Turn redundant phi elimination into proper HPhase.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/18549004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15533 ce2b1a6d-e550-0410-aec6-3dcde31c8c00