MIPS: Reland and fix "Fix LoadFieldByIndex to take mutable heap-numbers into account."

Port r20358 (5e9b653d)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/219163010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20369 ce2b1a6d-e550-0410-aec6-3dcde31c8c00