Eliminate gather-scatter-info offset_dt member

Eliminate gather-scatter-info offset_dt member

The following removes this only set member.  Sligthly complicated
by the hoops get_group_load_store_type jumps through.  I've simplified
that, noting the offset vector type that's relevant is that of the
actual offset SLP node, not of what vect_check_gather_scatter (re-)computes.

	* tree-vectorizer.h (gather_scatter_info::offset_dt): Remove.
	* tree-vect-data-refs.cc (vect_describe_gather_scatter_call):
	Do not set it.
	(vect_check_gather_scatter): Likewise.
	* tree-vect-stmts.cc (vect_truncate_gather_scatter_offset):
	Likewise.
	(get_group_load_store_type): Use the vector type of the offset
	SLP child.  Do not re-check vect_is_simple_use validated by
	SLP build.