[AMDGPU] Drop const for value that is copied (NFC_.

[fhahn]

Summary

This fixes

warning: loop variable 'Def' of type 'const llvm::Register' creates a copy from type 'const llvm::Register' [-Wrange-loop-analysis]

llvm::Register just contains a single unsigned and should be copied.

llvm/lib/Target/AMDGPU/SIPostRABundler.cpp

 bool SIPostRABundler::isDependentLoad(const MachineInstr &MI) const {
   if (!MI.mayLoad())
     return false;
 
   for (const MachineOperand &Op : MI.explicit_operands()) {
     if (!Op.isReg())
       continue;
     Register Reg = Op.getReg();
-    for (const Register Def : Defs)
+    for (Register Def : Defs)
       if (TRI->regsOverlap(Reg, Def))
         return true;
   }
 
   return false;
 }
 
 bool SIPostRABundler::runOnMachineFunction(MachineFunction &MF) {