store-merging: Handle vector CONSTRUCTORs using bswap [PR96239]

store-merging: Handle vector CONSTRUCTORs using bswap [PR96239]

I've tried to add such helper, but handling over just analysis and letting
each pass handle it differently seems complicated given the limitations of
the bswap infrastructure.

So, this patch just hooks the optimization also into store-merging so that
the original testcase from the PR can be fixed.

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96239
	* gimple-ssa-store-merging.c (maybe_optimize_vector_constructor): New
	function.
	(get_status_for_store_merging): Don't return BB_INVALID for blocks
	with potential bswap optimizable CONSTRUCTORs.
	(pass_store_merging::execute): Optimize vector CONSTRUCTORs with bswap
	if possible.

	* gcc.dg/tree-ssa/pr96239.c: New test.