[mips] Fix microMIPS unconditional branch offset handling

MipsSEInstrInfo class defines for internal purpose unconditional
branches as Mips::B nad Mips:J even in case of microMIPS code
generation. Under some conditions that leads to the bug - for rather long
branch which fits to Mips jump instruction offset size, but does not fit
to microMIPS jump offset size, we generate 'short' branch and later show
an error 'out of range PC16 fixup' after check in the isBranchOffsetInRange
routine.

Differential revision: https://reviews.llvm.org/D50615

llvm-svn: 340932