Fix MIPS code generation for mul by const
Fix MIPS code generation for mul by const

Looks like GNU as swallows "mul reg1, reg2, const" and transforms it
into something like "li r1, const" + "mul reg1, reg2, r1" and I've been
missing this fact until now.

JianpingZeng/SmallerC,JianpingZeng/SmallerC,gdos/SmallerC,gdos/SmallerC