MIPS: Tighten the code for MacroAssembler::ThrowUncatchable.

MIPS: Tighten the code for MacroAssembler::ThrowUncatchable.

Port r9909 (56c8728aa).

Original commit message:
Test at the bottom in the unwind loop.  Eliminate the possibility of a
useless move to the eax/rax/r0 register (currently impossible because
this function has two call sites).  Do not explicitly zero the context
because we've already saved 0 as the context in the handler.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8507007
Patch from Gergely Kis <gergely@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9923 ce2b1a6d-e550-0410-aec6-3dcde31c8c00