Fix type mismatch in SLPed constructors

Having the "same" vector types with different modes means that we can
end up vectorising a constructor with a different mode from the lhs.
This patch adds a VIEW_CONVERT_EXPR in that case.

This showed up on existing tests when testing with fixed-length
-msve-vector-bits=128.

2020-01-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-slp.c (vectorize_slp_instance_root_stmt): Use a
	VIEW_CONVERT_EXPR if the vectorized constructor has a diffeent
	type from the lhs.

gcc/ChangeLog

+2020-01-15  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* tree-vect-slp.c (vectorize_slp_instance_root_stmt): Use a
+	VIEW_CONVERT_EXPR if the vectorized constructor has a diffeent
+	type from the lhs.
+
 2020-01-15  Martin Liska  <mliska@suse.cz>
 
 	* ipa-profile.c (ipa_profile_read_edge_summary): Do not allow

gcc/tree-vect-slp.c

@@ -4307,6 +4307,10 @@ vectorize_slp_instance_root_stmt (slp_tree node, slp_instance instance)
 	{
 	  tree vect_lhs = gimple_get_lhs (child_stmt_info->stmt);
 	  tree root_lhs = gimple_get_lhs (instance->root_stmt->stmt);
+	  if (!useless_type_conversion_p (TREE_TYPE (root_lhs),
+					  TREE_TYPE (vect_lhs)))
+	    vect_lhs = build1 (VIEW_CONVERT_EXPR, TREE_TYPE (root_lhs),
+			       vect_lhs);
 	  rstmt = gimple_build_assign (root_lhs, vect_lhs);
 	  break;
 	}