[NFC] Use references to avoid copying (#99863)

Modifying `auto` to `auto&` to avoid unnecessary copying

Author: pratyay-p

llvm/include/llvm/CodeGen/MachineInstrBuilder.h

@@ -537,10 +537,10 @@ MachineInstr *buildDbgValueForSpill(MachineBasicBlock &BB,
                                     MachineBasicBlock::iterator I,
                                     const MachineInstr &Orig, int FrameIndex,
                                     Register SpillReg);
-MachineInstr *
-buildDbgValueForSpill(MachineBasicBlock &BB, MachineBasicBlock::iterator I,
-                      const MachineInstr &Orig, int FrameIndex,
-                      SmallVectorImpl<const MachineOperand *> &SpilledOperands);
+MachineInstr *buildDbgValueForSpill(
+    MachineBasicBlock &BB, MachineBasicBlock::iterator I,
+    const MachineInstr &Orig, int FrameIndex,
+    const SmallVectorImpl<const MachineOperand *> &SpilledOperands);
 
 /// Update a DBG_VALUE whose value has been spilled to FrameIndex. Useful when
 /// modifying an instruction in place while iterating over a basic block.

llvm/lib/CodeGen/MachineInstr.cpp

@@ -2296,9 +2296,9 @@ MachineInstrBuilder llvm::BuildMI(MachineBasicBlock &BB,
 
 /// Compute the new DIExpression to use with a DBG_VALUE for a spill slot.
 /// This prepends DW_OP_deref when spilling an indirect DBG_VALUE.
-static const DIExpression *
-computeExprForSpill(const MachineInstr &MI,
-                    SmallVectorImpl<const MachineOperand *> &SpilledOperands) {
+static const DIExpression *computeExprForSpill(
+    const MachineInstr &MI,
+    const SmallVectorImpl<const MachineOperand *> &SpilledOperands) {
   assert(MI.getDebugVariable()->isValidLocationForIntrinsic(MI.getDebugLoc()) &&
          "Expected inlined-at fields to agree");
 
@@ -2353,7 +2353,7 @@ MachineInstr *llvm::buildDbgValueForSpill(MachineBasicBlock &BB,
 MachineInstr *llvm::buildDbgValueForSpill(
     MachineBasicBlock &BB, MachineBasicBlock::iterator I,
     const MachineInstr &Orig, int FrameIndex,
-    SmallVectorImpl<const MachineOperand *> &SpilledOperands) {
+    const SmallVectorImpl<const MachineOperand *> &SpilledOperands) {
   const DIExpression *Expr = computeExprForSpill(Orig, SpilledOperands);
   MachineInstrBuilder NewMI =
       BuildMI(BB, I, Orig.getDebugLoc(), Orig.getDesc());

llvm/lib/CodeGen/RegAllocFast.cpp

@@ -584,7 +584,7 @@ void RegAllocFastImpl::spill(MachineBasicBlock::iterator Before,
       SpilledOperandsMap;
   for (MachineOperand *MO : LRIDbgOperands)
     SpilledOperandsMap[MO->getParent()].push_back(MO);
-  for (auto MISpilledOperands : SpilledOperandsMap) {
+  for (const auto &MISpilledOperands : SpilledOperandsMap) {
     MachineInstr &DBG = *MISpilledOperands.first;
     // We don't have enough support for tracking operands of DBG_VALUE_LISTs.
     if (DBG.isDebugValueList())