tree-optimization/117080 - Add SLP_TREE_MEMORY_ACCESS_TYPE

tree-optimization/117080 - Add SLP_TREE_MEMORY_ACCESS_TYPE

It turns out target costing code looks at STMT_VINFO_MEMORY_ACCESS_TYPE
to identify operations from (emulated) gathers for example.  This
doesn't work for SLP loads since we do not set STMT_VINFO_MEMORY_ACCESS_TYPE
there as the vectorization strathegy might differ between different
stmt uses.  It seems we got away with setting it for stores though.
The following adds a memory_access_type field to slp_tree and sets it
from load and store vectorization code.  All the costing doesn't record
the SLP node (that was only done selectively for some corner case).  The
costing is really in need of a big overhaul, the following just massages
the two relevant ops to fix gcc.dg/target/pr88531-2[bc].c FAILs when
switching on SLP for non-grouped stores.  In particular currently
we either have a SLP node or a stmt_info in the cost hook but not both.

So the following mitigates this, postponing a rewrite of costing to
next stage1.  Other targets look possibly affected as well but are
left to respective maintainers to update.

	PR tree-optimization/117080
	* tree-vectorizer.h (_slp_tree::memory_access_type): Add.
	(SLP_TREE_MEMORY_ACCESS_TYPE): New.
	(record_stmt_cost): Add another overload.
	* tree-vect-slp.cc (_slp_tree::_slp_tree): Initialize
	memory_access_type.
	* tree-vect-stmts.cc (vectorizable_store): Set
	SLP_TREE_MEMORY_ACCESS_TYPE.
	(vectorizable_load): Likewise.  Also record the SLP node
	when costing emulated gather offset decompose and vector
	composition.
	* config/i386/i386.cc (ix86_vector_costs::add_stmt_cost): Also
	recognize SLP emulated gather/scatter.