[mips][microMIPS] This patch implements functionality in MIPS delay slot
filler such as if delay slot filler have to put NOP instruction into the
delay slot of microMIPS BEQ or BNE instruction which uses the register $0,
then instead of emitting NOP this instruction is replaced by the corresponding
microMIPS compact branch instruction, i.e. BEQZC or BNEZC.

Differential Revision: http://reviews.llvm.org/D3566