vect: Fix multi-vector SLP gather loads [PR103744]

vect: Fix multi-vector SLP gather loads [PR103744]

This PR shows that I didn't properly test the multi-vector case when
adding support for SLP gather loads.  The patch fixes that case using
the same approach as we do for non-SLP cases: keep the scalar base
the same, but iterate through the (also multi-vector) vector offsets.
“vec_num * j + i” is already used elsewhere as a way of handling both
the multi-vector SLP case and the multi-vector non-SLP case.

gcc/
	PR tree-optimization/103744
	* tree-vect-stmts.c (vectorizable_load): Handle multi-vector
	SLP gather loads.

gcc/testsuite/
	PR tree-optimization/103744
	* gcc.dg/vect/pr103744-1.c: New test.
	* gcc.dg/vect/pr103744-2.c: Likewise.