MIPS: Allocate generator result objects before unwinding try handlers

Port r15079 (1ebdfdc6)

Original commit message:
When a generator suspends, it saves its state out to the heap and
unwinds try handlers but doesn't pop anything off the stack.  Instead it
relies on no GC happening between the suspend and the return from the
generator.  However this was not the case: boxing the result object
could cause GC, which would try to traverse the stack but would
misinterpret words from unwound try handlers as heap objects.

This CL changes to allocate the result objects before the suspend.  It
also removes the generators-iteration skip introduced in r15065.

BUG=

Review URL: https://codereview.chromium.org/16820004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15096 ce2b1a6d-e550-0410-aec6-3dcde31c8c00