[MIPS GlobalISel] Select branch instructions

Select G_BR and G_BRCOND for MIPS32.
Unconditional branch G_BR does not have register operand,
for that reason we only add tests.
Since conditional branch G_BRCOND compares register to zero on MIPS32,
explicit extension must be performed on i1 condition in order to set
high bits to appropriate value.

Differential Revision: https://reviews.llvm.org/D58182

llvm-svn: 354022