[AArch64] Fix predicate alignment for fixed-length SVE

aarch64_simd_vector_alignment was only giving predicates 16-bit
alignment in VLA mode, not VLS mode.  I think the problem is latent
because we can't yet create an ABI predicate type, but it seemed worth
fixing in a standalone patch rather than as part of the main ACLE series.

The ACLE patches have tests for this.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_simd_vector_alignment): Return
	16 for SVE predicates even if they are fixed-length.

From-SVN: r274522

gcc/ChangeLog

 2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* config/aarch64/aarch64.c (aarch64_simd_vector_alignment): Return
+	16 for SVE predicates even if they are fixed-length.
+
+2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* config/aarch64/aarch64-sve.md (and<PRED_ALL:mode>3): Make the
 	operand order match the MOV /Z alias.
 

gcc/config/aarch64/aarch64.c

@@ -15915,11 +15915,13 @@ aarch64_simd_attr_length_rglist (machine_mode mode)
 static HOST_WIDE_INT
 aarch64_simd_vector_alignment (const_tree type)
 {
+  /* ??? Checking the mode isn't ideal, but VECTOR_BOOLEAN_TYPE_P can
+     be set for non-predicate vectors of booleans.  Modes are the most
+     direct way we have of identifying real SVE predicate types.  */
+  if (GET_MODE_CLASS (TYPE_MODE (type)) == MODE_VECTOR_BOOL)
+    return 16;
   if (TREE_CODE (TYPE_SIZE (type)) != INTEGER_CST)
-    /* ??? Checking the mode isn't ideal, but VECTOR_BOOLEAN_TYPE_P can
-       be set for non-predicate vectors of booleans.  Modes are the most
-       direct way we have of identifying real SVE predicate types.  */
-    return GET_MODE_CLASS (TYPE_MODE (type)) == MODE_VECTOR_BOOL ? 16 : 128;
+    return 128;
   return wi::umin (wi::to_wide (TYPE_SIZE (type)), 128).to_uhwi ();
 }