[mips] Print instructions "beq", "bne" and "or" using assembler pseudo

instructions "beqz", "bnez" and "move", when possible.

beq $2, $zero, $L1 => beqz $2, $L1
bne $2, $zero, $L1 => bnez $2, $L1
or  $2, $3, $zero  => move $2, $3

llvm-svn: 187229