Fix PR78556 - left shift of negative values

Running bootstrap-ubsan on ppc64le shows many instances of e.g.:
 config/rs6000/rs6000.c:6217:36: runtime error: left shift of negative value -12301

        PR target/78556
	* config/rs6000/rs6000.c (vspltis_constant): Add casts to avoid
	left shifting of negative values.

From-SVN: r242928

gcc/ChangeLog

+2016-11-28  Markus Trippelsdorf  <markus@trippelsdorf.de>
+
+	PR target/78556
+	* config/rs6000/rs6000.c (vspltis_constant): Add casts to avoid
+	left shifting of negative values.
+
 2016-11-28  Jakub Jelinek  <jakub@redhat.com>
 
 	PR fortran/78298

gcc/config/rs6000/rs6000.c

@@ -6214,7 +6214,9 @@ vspltis_constant (rtx op, unsigned step, unsigned copies)
       bitsize /= 2;
       small_val = splat_val >> bitsize;
       mask >>= bitsize;
-      if (splat_val != ((small_val << bitsize) | (small_val & mask)))
+      if (splat_val != ((HOST_WIDE_INT)
+          ((unsigned HOST_WIDE_INT) small_val << bitsize)
+          | (small_val & mask)))
 	return false;
       splat_val = small_val;
     }