[mips] Handle UImm16_AltRelaxed match type

[abeserminji]

Summary

Currently, UImm16_AltRelaxed match type is not handled in MatchAndEmitInstruction(...) function, which may result in llvm_unreachable() behavior.
This patch adds necessary case for this match type.

[sdardis]

Hm, as far as I can see is what's currently happening--with dati $4, $4, 0x1FFFF for mips64r6 for example--is that the matcher is picking the mips64r6 instruction first, fails to parse it as the third operand is out of range, attempts to parse 'dati $4, $4, 0x1FFFF' as a microMIPS64R6 instruction, fails again as the third operand is out of range but returns an error based on the last mnemonic matched despite that mnemonic being invalid as the target is mips64r6 and not in micromips mode. Since the microMIPS64r6 and MIPS64r6 instruction definitions disagree with each other, we don't get the expected match result, nor do we hit llvm_unreachable due to the error arising from attempting to match that instruction to the microMIPS64r6 instruction definition.

No need for a test case since as far as I can see it's impossible to hit the error path without fixing the microMIPS64r6 instruction definitions, but we're deprecating that backend anyway.

LGTM.

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

bool MipsAsmParser::MatchAndEmitInstruction(SMLoc IDLoc, unsigned &Opcode,
   case Match_SImm10_0:
     return Error(RefineErrorLoc(IDLoc, Operands, ErrorInfo),
                  "expected 10-bit signed immediate");
   case Match_SImm11_0:
     return Error(RefineErrorLoc(IDLoc, Operands, ErrorInfo),
                  "expected 11-bit signed immediate");
   case Match_UImm16:
   case Match_UImm16_Relaxed:
+  case Match_UImm16_AltRelaxed:
     return Error(RefineErrorLoc(IDLoc, Operands, ErrorInfo),
                  "expected 16-bit unsigned immediate");
   case Match_SImm16:
   case Match_SImm16_Relaxed:
     return Error(RefineErrorLoc(IDLoc, Operands, ErrorInfo),
                  "expected 16-bit signed immediate");
   case Match_SImm19_Lsl2:
     return Error(RefineErrorLoc(IDLoc, Operands, ErrorInfo),