tree-optimization/116573 - .SELECT_VL for SLP

tree-optimization/116573 - .SELECT_VL for SLP

The following restores the use of .SELECT_VL for testcases where it
is safe to use even when using SLP.  I've for now restricted it
to single-lane SLP plus optimistically allow store-lane nodes
and assume single-lane roots are not widened but at most to
load-lane who should be fine.

	PR tree-optimization/116573
	* tree-vect-loop.cc (vect_analyze_loop_2): Allow .SELECV_VL
	for SLP but disable it when there's multi-lane instances.
	* tree-vect-stmts.cc (vectorizable_store): Only compute the
	ptr increment when generating code.
	(vectorizable_load): Likewise.