tree-optimization/116575 - handle SLP of permuted masked loads

The following handles SLP discovery of permuted masked loads which
was prohibited (because wrongly handled) for PR114375.  In particular
with single-lane SLP at the moment all masked group loads appear
permuted and we fail to use masked load lanes as well.  The following
addresses parts of the issues, starting with doing correct basic
discovery - namely discover an unpermuted mask load followed by
a permute node.  In particular groups with gaps do not support masking
yet (and didn't before w/o SLP IIRC).  There's still issues with
how we represent masked load/store-lanes I think, but I first have to
get my hands on a good testcase.

	PR tree-optimization/116575
	PR tree-optimization/114375
	* tree-vect-slp.cc (vect_build_slp_tree_2): Do not reject
	permuted mask loads without gaps but instead discover a
	node for the full unpermuted load and permute that with
	a VEC_PERM node.

	* gcc.dg/vect/vect-pr114375.c: Expect vectorization now with avx2.