Refactor x86 decl based scatter vectorization, prepare SLP

Refactor x86 decl based scatter vectorization, prepare SLP

The following refactors the x86 decl based scatter vectorization
similar to what I did to the gather path.  This prepares scatters
for SLP as well, mainly single-lane since there are multiple
missing bits to support multi-lane scatters.

Tested extensively on the SLP-only branch which has the ability
to force SLP even for single lanes.

	PR tree-optimization/111133
	* tree-vect-stmts.cc (vect_build_scatter_store_calls):
	Remove and refactor to ...
	(vect_build_one_scatter_store_call): ... this new function.
	(vectorizable_store): Use vect_check_scalar_mask to record
	the SLP node for the mask operand.  Code generate scatters
	with builtin decls from the main scatter vectorization
	path and prepare that for SLP.
	* tree-vect-slp.cc (vect_get_operand_map): Do not look
	at the VDEF to decide between scatter or gather since that
	doesn't work for patterns.  Use the LHS being an SSA_NAME
	or not instead.