[mips] Make simm6 consistent with the rest. NFC.

[dsanders]

Summary

Depends on D18145

[vkalintiris]

LGTM.

lib/Target/Mips/MipsInstrInfo.td

 }
 def calltarget  : Operand<iPTR> {
   let EncoderMethod = "getJumpTargetOpValue";
   let ParserMatchClass = MipsJumpTargetAsmOperand;
 }
 
 def imm64: Operand<i64>;
 
-def simm6 : Operand<i32> {
-  let ParserMatchClass = ConstantSImm6AsmOperandClass;
-  let OperandType = "OPERAND_IMMEDIATE";
-}
 def simm9 : Operand<i32>;
 def simm10 : Operand<i32>;
 def simm11 : Operand<i32>;
 
 def simm16      : Operand<i32> {
   let DecoderMethod= "DecodeSimm16";
 }
 

 foreach I = {1, 2, 3, 4, 5, 6, 8} in
   def vsplat_uimm # I : Operand<vAny> {
     let PrintMethod = "printUImm<" # I # ">";
     let ParserMatchClass =
         !cast<AsmOperandClass>("ConstantUImm" # I # "AsmOperandClass");
   }
 
 // Signed operands
-foreach I = {4, 5} in
+foreach I = {4, 5, 6} in
   def simm # I : Operand<i32> {
     let DecoderMethod = "DecodeSImmWithOffsetAndScale<" # I # ">";
     let ParserMatchClass =
         !cast<AsmOperandClass>("ConstantSImm" # I # "AsmOperandClass");
   }
 
 def simm7_lsl2 : Operand<OtherVT> {
   let EncoderMethod = "getSImm7Lsl2Encoding";