[mips] Map SW instruction to its microMIPS R6 variant
To provide mapping between standard and microMIPS R6 variants of the
sw command we have to rename SWSP_xxx commands from "sw" to "swsp".
Otherwise tablegen starts to show the error `Multiple matches found
for SW'. After that to restore printing SWSP command as sw, I add
an appropriate MipsInstAlias instance.
We also need to implement "size reduction" for microMIPS R6. But this
task is for separate patch. After that the micromips-lwsp-swsp.ll test
case will be extended.
Differential Revision: http://reviews.llvm.org/D59046