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.