[rs6000] Fix PR92566 by checking VECTOR_UNIT_NONE_P

As Segher pointed out in PR92566, we shouldn't offer some vector modes which
aren't supported under current setting.  This patch is to make it check by
VECTOR_UNIT_NONE_P which is initialized as current architecture masks.

2019-11-28  Kewen Lin  <linkw@gcc.gnu.org>

    PR target/92566
    * gcc/config/rs6000/rs6000.c (rs6000_preferred_simd_mode): Check by
    VECTOR_UNIT_NONE_P instead.

From-SVN: r278800

gcc/ChangeLog

+2019-11-28  Kewen Lin  <linkw@gcc.gnu.org>
+
+	PR target/92566
+	* gcc/config/rs6000/rs6000.c (rs6000_preferred_simd_mode): Check
+	by VECTOR_UNIT_NONE_P instead.
+
 2019-11-28  Hongtao Liu  <hongtao.liu@inte.com>
 
 	* gcc/config/i386/sse.md (avx512f_maskcmp<mode>3):

gcc/config/rs6000/rs6000.c

@@ -4915,30 +4915,11 @@ rs6000_builtin_vectorization_cost (enum vect_cost_for_stmt type_of_cost,
 static machine_mode
 rs6000_preferred_simd_mode (scalar_mode mode)
 {
-  if (TARGET_VSX)
-    switch (mode)
-      {
-      case E_DFmode:
-	return V2DFmode;
-      default:;
-      }
-  if (TARGET_ALTIVEC || TARGET_VSX)
-    switch (mode)
-      {
-      case E_SFmode:
-	return V4SFmode;
-      case E_TImode:
-	return V1TImode;
-      case E_DImode:
-	return V2DImode;
-      case E_SImode:
-	return V4SImode;
-      case E_HImode:
-	return V8HImode;
-      case E_QImode:
-	return V16QImode;
-      default:;
-      }
+  opt_machine_mode vmode = mode_for_vector (mode, 16 / GET_MODE_SIZE (mode));
+
+  if (vmode.exists () && !VECTOR_UNIT_NONE_P (vmode.require ()))
+    return vmode.require ();
+
   return word_mode;
 }