ARM: Add a postRA pass to legalize stack offsets. Greedy approach (reserve IP).

src/IceOperand.h

Index: src/IceOperand.h
diff --git a/src/IceOperand.h b/src/IceOperand.h
index 8db3cf1187b630e35af9a95ed546372d0aa35005..90c80eb9e72e8f1c43502dae2205e35c0edd4c96 100644
--- a/src/IceOperand.h
+++ b/src/IceOperand.h
@@ -503,6 +503,9 @@ public:
   using Operand::dump;
   void dump(const Cfg *Func, Ostream &Str) const override;
 
+  /// Return reg num of base register, if different from stack/frame register.
+  virtual int32_t getBaseRegNum() const { return NoRegister; }
+
   static bool classof(const Operand *Operand) {
     OperandKind Kind = Operand->getKind();
     return Kind >= kVariable && Kind <= kVariable_Num;