MIPS: Improve code aging sequence.

Port r15697 (61d56196)

Original commit message:
The code aging sequence contains a load which is unecessary for optimised
function. This has been replaced by a nop.

BUG=

Review URL: https://codereview.chromium.org/19683005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15733 ce2b1a6d-e550-0410-aec6-3dcde31c8c00