Subzero: Prototype to make use of RegNumT::No Register more concise

src/IceInstARM32.h

Index: src/IceInstARM32.h
diff --git a/src/IceInstARM32.h b/src/IceInstARM32.h
index 5d53afd0cd5281fb0ae5b24ca2ec395b4b91b374..c939246f2b3991590c441ece325f43cbea2df291 100644
--- a/src/IceInstARM32.h
+++ b/src/IceInstARM32.h
@@ -361,7 +361,7 @@ public:
 private:
   StackVariable(Type Ty, SizeT Index)
       : Variable(StackVariableKind, Ty, Index) {}
-  RegNumT BaseRegNum = RegNumT::NoRegister;
+  RegNumT BaseRegNum;
 };
 
 /// Base class for ARM instructions. While most ARM instructions can be