MIPS: Fix assignment of function name constant.

Port r19379 (622d830)

Original commit message:
If it's shadowed by a variable of the same name and both are forcibly
context-allocated, the function is assigned to the wrong context slot.

BUG=v8:3138
LOG=Y
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/166813002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19391 ce2b1a6d-e550-0410-aec6-3dcde31c8c00