[AArch64] Don't apply mode_for_int_vector to scalars

aarch64_emit_approx_sqrt handles both vectors and scalars and was using
mode_for_int_vector even for the scalar case.  Although that happened
to work, it isn't how mode_for_int_vector is supposed to be used.

2019-10-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_emit_approx_sqrt): Use
	int_mode_for_mode rather than mode_for_int_vector for scalars.

From-SVN: r277311

gcc/ChangeLog

+2019-10-23  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* config/aarch64/aarch64.c (aarch64_emit_approx_sqrt): Use
+	int_mode_for_mode rather than mode_for_int_vector for scalars.
+
 2019-10-23  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/92179

gcc/config/aarch64/aarch64.c

@@ -11828,7 +11828,9 @@ aarch64_emit_approx_sqrt (rtx dst, rtx src, bool recp)
     /* Caller assumes we cannot fail.  */
     gcc_assert (use_rsqrt_p (mode));
 
-  machine_mode mmsk = mode_for_int_vector (mode).require ();
+  machine_mode mmsk = (VECTOR_MODE_P (mode)
+		       ? mode_for_int_vector (mode).require ()
+		       : int_mode_for_mode (mode).require ());
   rtx xmsk = gen_reg_rtx (mmsk);
   if (!recp)
     /* When calculating the approximate square root, compare the