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

src/IceRegistersX8632.h

 //===- subzero/src/IceRegistersX8632.h - Register information ---*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 62 matching lines @@
 
   /// An enum of X87 Stack Registers. The enum value does match the encoding
   /// used to binary encode register operands in instructions.
   enum X87STRegister {
 #define X(val, encode, name) Encoded_##val = encode,
     X87ST_REGX8632_TABLE
 #undef X
         Encoded_Not_X87STReg = -1
   };
 
-  static inline X87STRegister getEncodedSTReg(int32_t RegNum) {
-    assert(Encoded_X87ST_First <= RegNum);
-    assert(RegNum <= Encoded_X87ST_Last);
-    return X87STRegister(RegNum);
+  static inline X87STRegister getEncodedSTReg(uint32_t X87RegNum) {
+    assert(int(Encoded_X87ST_First) <= int(X87RegNum));
+    assert(X87RegNum <= Encoded_X87ST_Last);
+    return X87STRegister(X87RegNum);
   }
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEREGISTERSX8632_H