tree-optimization/101121 - avoid infinite SLP build

tree-optimization/101121 - avoid infinite SLP build

The following plugs another hole where we cache a failed SLP build
attempt with an all-success 'matches'.  It also adds checking that
we don't do that.

2021-06-21  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101121
	* tree-vect-slp.c (vect_build_slp_tree_2): To not fail fatally
	when we just lack a stmt with the desired op when doing permutation.
	(vect_build_slp_tree): When caching a failed SLP build attempt
	assert that at least one lane is marked as not matching.

	* gfortran.dg/pr101121.f: New testcase.