CodeGen: Move undef_tied_input declaration

This doesn't belong in ARM specific code since it's generally
recognized by tablegen.

llvm/include/llvm/Target/Target.td

@@ -900,6 +900,10 @@ def untyped_imm_0 : TypedOperand<"OPERAND_GENERIC_IMM_0"> {
 ///
 def zero_reg;
 
+/// undef_tied_input - Special node to indicate an input register tied
+/// to an output which defaults to IMPLICIT_DEF.
+def undef_tied_input;
+
 /// All operands which the MC layer classifies as predicates should inherit from
 /// this class in some manner. This is already handled for the most commonly
 /// used PredicateOperand, but may be useful in other circumstances.

llvm/lib/Target/ARM/ARMInstrFormats.td

@@ -205,7 +205,6 @@ def VPTPredROperand : AsmOperandClass {
   let Name = "VPTPredR";
   let PredicateMethod = "isVPTPred";
 }
-def undef_tied_input;
 
 // Operand classes for the cluster of MC operands describing a
 // VPT-predicated MVE instruction.