i386: Fix unprotected REGNO in aeswidekl_operation

I hit an ICE in aeswidekl_operation while testing the late-combine
pass on x86.  The predicate tested REGNO without first testing REG_P.

gcc/
	* config/i386/predicates.md (aeswidekl_operation): Protect
	REGNO check with REG_P.

gcc/config/i386/predicates.md

@@ -2260,6 +2260,7 @@
 	  || GET_CODE (SET_SRC (elt)) != UNSPEC_VOLATILE
 	  || GET_MODE (SET_SRC (elt)) != V2DImode
 	  || XVECLEN (SET_SRC (elt), 0) != 1
+	  || !REG_P (XVECEXP (SET_SRC (elt), 0, 0))
 	  || REGNO (XVECEXP (SET_SRC (elt), 0, 0)) != GET_SSE_REGNO (i))
 	return false;
     }