[mips][sched] Split IIBranch into specific instruction classes.

Summary:
Almost no functional change since the InstrItinData's have been duplicated.
The one functional change is to remove IIBranch from the MSA branches. The
classes will be assigned to the MSA instructions as part of implementing
the P5600 scheduler.

II_IndirectBranchPseudo and II_ReturnPseudo can probably be removed. I've
preserved the itinerary information for the corresponding pseudo
instructions to avoid making a functional change to these pseudos in
this patch.

Reviewers: vkalintiris

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12189

llvm-svn: 248273