vect: Fix mask handling for SLP gathers [PR103761]

vect: Fix mask handling for SLP gathers [PR103761]

check_load_store_for_partial_vectors predates the support for SLP
gathers and so had a hard-coded assumption that gathers/scatters
(and load/stores lanes) would be non-SLP operations.  This patch
passes down the slp_node so that the routine can work out how
many vectors are needed in both the SLP and non-SLP cases.

gcc/
	PR tree-optimization/103761
	* tree-vect-stmts.cc (check_load_store_for_partial_vectors): Replace
	the ncopies parameter with an slp_node parameter.  Calculate the
	number of vectors based on it and vectype.  Rename lambda to
	group_memory_nvectors.
	(vectorizable_store, vectorizable_load): Update calls accordingly.

gcc/testsuite/
	PR tree-optimization/103761
	* gcc.dg/vect/pr103761.c: New test.
	* gcc.target/aarch64/sve/pr103761.c: Likewise.