Subzero: Use a proper RegNumT type instead of int32_t/SizeT.

src/IceInstARM32.h

Index: src/IceInstARM32.h
diff --git a/src/IceInstARM32.h b/src/IceInstARM32.h
index 4d8f02b5b712e21d1f6ddf0a88f6cce172cfe286..40518c141c17394e52fcac80912539fcde2488c2 100644
--- a/src/IceInstARM32.h
+++ b/src/IceInstARM32.h
@@ -354,14 +354,14 @@ public:
   static bool classof(const Operand *Operand) {
     return Operand->getKind() == StackVariableKind;
   }
-  void setBaseRegNum(int32_t RegNum) { BaseRegNum = RegNum; }
-  int32_t getBaseRegNum() const override { return BaseRegNum; }
+  void setBaseRegNum(RegNumT RegNum) { BaseRegNum = RegNum; }
+  RegNumT getBaseRegNum() const override { return BaseRegNum; }
   // Inherit dump() and emit() from Variable.
 
 private:
   StackVariable(Type Ty, SizeT Index)
       : Variable(StackVariableKind, Ty, Index) {}
-  int32_t BaseRegNum = Variable::NoRegister;
+  RegNumT BaseRegNum = RegNumT::NoRegister;
 };
 
 /// Base class for ARM instructions. While most ARM instructions can be