[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

llvm-svn: 356045