The patch adds CodeGen support for microMIPS32r6 ADD, ADDIU*, ADDU* and microMIPS64r6 DADD* instructions:
- updated add.ll with tests for microMIPSr6 ADD, ADDIU*, ADDU* and DADD* instruction selection
- separated microMIPS64r6 DADD* instructions from equivalent MIPS64 instructions using NotInMicroMips predicate
- added DAG pattern to ADDIU description class for proper selection of the instruction
- implemented DADD, DADDIU and DADDU instructions for microMIPS64r6 and added tests for the standard encodings