Subzero. Removes references to %ah.

src/IceRegistersX8664.h

Index: src/IceRegistersX8664.h
diff --git a/src/IceRegistersX8664.h b/src/IceRegistersX8664.h
index 3e4c868ddfbb5898125bc5a88a9d832883cebc0c..25b9516b93000610b0bea41e50193803d5adc822 100644
--- a/src/IceRegistersX8664.h
+++ b/src/IceRegistersX8664.h
@@ -79,10 +79,7 @@ public:
   }
 
   static inline ByteRegister getEncodedByteReg(int32_t RegNum) {
-    // In x86-64, AH is not encodable when the REX prefix is used; the same
-    // encoding is used for spl. Therefore, ah needs special handling.
-    if (RegNum == Reg_ah)
-      return Encoded_Reg_spl;
+    assert(Reg_GPR_First <= RegNum && RegNum <= Reg_GPR_Last);
     return ByteRegister(RegNum - Reg_GPR_First);
   }