MIPS: Convert ToBooleanStub to a HydrogenStub. Currently just using the existing HBranch instruction, which is still fully implemented in Lithium. Will refactor HBranch in a next CL.

Port r14886 (068e9135)

BUG=

Review URL: https://codereview.chromium.org/15664008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14893 ce2b1a6d-e550-0410-aec6-3dcde31c8c00