MIPS: Delegating yield does not re-box result objects.

Port r15113 (9d28761)

Delegating yield (yield*) should just pass on the iterator results it
receives instead of re-boxing them.

TEST=mjsunit/harmony/generators-iteration
BUG=

Review URL: https://codereview.chromium.org/16855009
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15131 ce2b1a6d-e550-0410-aec6-3dcde31c8c00