MIPS: Fix stack frame reconstruction for generators with formal arguments.

Port r15230 (e2bbe01)

Original commit message:
The formal parameter count was always being treated as an untagged
integer, but it is actually a Smi on ia32 and arm.

BUG=v8:2355
TEST=mjsunit/harmony/generators-iteration

Review URL: https://codereview.chromium.org/17261028
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15250 ce2b1a6d-e550-0410-aec6-3dcde31c8c00