refactor(evm): remove unused PhysicalReg from Variable class

The PhysicalReg field and related methods (setPhysicalRegister,
getPhysicalRegister, hasPhysicalRegister) were added for a previous
approach to gas register allocation that used direct physical register
assignment. The current implementation uses explicit COPY instructions
instead, making these fields unused.

Author: abmcar

src/compiler/mir/variable.h

@@ -18,14 +18,9 @@ class Variable : public NonCopyable {
 
   VariableIdx getVarIdx() const { return VarIdx; }
 
-  void setPhysicalRegister(unsigned PhysReg) { PhysicalReg = PhysReg; }
-  unsigned getPhysicalRegister() const { return PhysicalReg; }
-  bool hasPhysicalRegister() const { return PhysicalReg != 0; }
-
 private:
   VariableIdx VarIdx;
   MType *Type = nullptr;
-  unsigned PhysicalReg = 0;
 };
 
 } // namespace COMPILER