tree-optimization/101120 - fix compile-time issue with SLP groups

tree-optimization/101120 - fix compile-time issue with SLP groups

This places two hacks to avoid an old compile-time issue when
vectorizing large permuted SLP groups with gaps where we end up
emitting loads and IV adjustments for the gap as well and those
have quite a high cost until they are eventually cleaned up.

The first hack is to fold the auto-inc style IV updates early
in the vectorizer rather than in the next forwprop pass which
shortens the SSA use-def chains of the used IV.

The second hack is to remove the unused loads after we've picked
all that we possibly use.

2021-06-18  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101120
	* tree-vect-data-refs.c (bump_vector_ptr): Fold the
	built increment.
	* tree-vect-slp.c (vect_transform_slp_perm_load): Add
	DR chain DCE capability.
	* tree-vectorizer.h (vect_transform_slp_perm_load): Adjust.
	* tree-vect-stmts.c (vectorizable_load): Remove unused
	loads in the DR chain for SLP.