MIPS: Refactor and improve inlined double-aligned allocations

Additionally, fix object size register clobbering in Allocate routine.

Based on 13284 (d42bf80).

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/22582002

Patch from Dusan Milosavljevic <Dusan.Milosavljevic@rt-rk.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16112 ce2b1a6d-e550-0410-aec6-3dcde31c8c00