Mips assembler: Add branch macro definitions

This patch adds bnez and beqz instructions which represent alias definitions for bne and beq instructions as follows:
bnez $rs,$imm => bne $rs,$zero,$imm
beqz $rs,$imm => beq $rs,$zero,$imm

The corresponding test cases are added.

Patch by Vladimir Medic

llvm-svn: 182040