MIPS: Fix flaky mjsunit/harmony/generators-iteration test failure.

This fixes the result allocation in EmitCreateIteratorResult,
when a GC is required. The Runtime call returns the allocated
pointer in v0 register, but the rest of the function was
expected it in a0 register. Refactored the function to use v0 for
the result in the whole function.

BUG=

Review URL: https://codereview.chromium.org/21244003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15965 ce2b1a6d-e550-0410-aec6-3dcde31c8c00