[mips] MIPS64R6 compact branch support

MIPS64R6 compact branch support. As the MIPS LLVM backend uses distinct
MachineInstrs for certain 32 and 64 bit instructions (e.g. BEQ & BEQ64) that
map to the same instruction, extend compact branch support for the
corresponding 64bit branches.

Reviewers: dsanders

Differential Revision: https://reviews.llvm.org/D20164

llvm-svn: 276739