[DebugInfo] Follow up c521e44def with an API improvement

As mentioned post-commit in D85749, the 'substituteDebugValuesForInst'
method added in c521e44def would be better off with a limit on the
number of operands to substitute. This handles the common case of
"substitute the first operand between these two differing instructions",
or possibly up to N first operands.

llvm/include/llvm/CodeGen/MachineFunction.h

@@ -458,7 +458,11 @@ public:
   /// which has the same signature (i.e., def operands in the same place) but
   /// a modified instruction type, flags, or otherwise. An example: X86 moves
   /// are sometimes transformed into equivalent LEAs.
-  void substituteDebugValuesForInst(const MachineInstr &Old, MachineInstr &New);
+  /// If the two instructions are not the same opcode, limit which operands to
+  /// examine for substitutions to the first N operands by setting
+  /// \p MaxOperand.
+  void substituteDebugValuesForInst(const MachineInstr &Old, MachineInstr &New,
+                                    unsigned MaxOperand = UINT_MAX);
 
   MachineFunction(Function &F, const LLVMTargetMachine &Target,
                   const TargetSubtargetInfo &STI, unsigned FunctionNum,

llvm/lib/CodeGen/MachineFunction.cpp

@@ -955,7 +955,8 @@ void MachineFunction::makeDebugValueSubstitution(DebugInstrOperandPair A,
 }
 
 void MachineFunction::substituteDebugValuesForInst(const MachineInstr &Old,
-                                                   MachineInstr &New) {
+                                                   MachineInstr &New,
+                                                   unsigned MaxOperand) {
   // If the Old instruction wasn't tracked at all, there is no work to do.
   unsigned OldInstrNum = Old.peekDebugInstrNum();
   if (!OldInstrNum)
@@ -965,12 +966,16 @@ void MachineFunction::substituteDebugValuesForInst(const MachineInstr &Old,
   // Avoid creating new instr numbers unless we create a new substitution.
   // While this has no functional effect, it risks confusing someone reading
   // MIR output.
+  // Examine all the operands, or the first N specified by the caller.
+  MaxOperand = std::min(MaxOperand, Old.getNumOperands());
   for (unsigned int I = 0; I < Old.getNumOperands(); ++I) {
     const auto &OldMO = Old.getOperand(I);
+    auto &NewMO = New.getOperand(I);
+    (void)NewMO;
 
     if (!OldMO.isReg() || !OldMO.isDef())
       continue;
-    assert(Old.getOperand(I).isDef());
+    assert(NewMO.isDef());
 
     unsigned NewInstrNum = New.getDebugInstrNum();
     makeDebugValueSubstitution(std::make_pair(OldInstrNum, I),