MIPS: Support for DoubleToIStub (truncating).

Port r16322 (8a03070)

Original commit message:
Added support for truncating DoubleToIStub and reorganize the macro-assembler
dToI operations to do the fast-path inline and the slow path by calling the
stub.

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/23694014

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16461 ce2b1a6d-e550-0410-aec6-3dcde31c8c00