tree-optimization/116575 - avoid ICE with SLP mask_load_lane

The following avoids performing re-discovery with single lanes in
the attempt to for the use of mask_load_lane as rediscovery will
fail since a single lane of a mask load will appear permuted which
isn't supported.

	PR tree-optimization/116575
	* tree-vect-slp.cc (vect_analyze_slp): Properly compute
	the mask argument for vect_load/store_lanes_supported.
	When the load is masked for now avoid rediscovery.

	* gcc.dg/vect/pr116575.c: New testcase.