MIPS: Replace LoadNamedFieldPolymorphic with explicit branches.

Port r16133 (dce32514)

BUG=

Review URL: https://codereview.chromium.org/22255005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16151 ce2b1a6d-e550-0410-aec6-3dcde31c8c00