vect: Misalign checks for gather/scatter.

vect: Misalign checks for gather/scatter.

This patch adds simple misalignment checks for gather/scatter
operations.  Previously, we assumed that those perform element accesses
internally so alignment does not matter.  The riscv vector spec however
explicitly states that vector operations are allowed to fault on
element-misaligned accesses.  Reasonable uarchs won't, but...

For gather/scatter we have two paths in the vectorizer:

 (1) Regular analysis based on datarefs.  Here we can also create
     strided loads.
 (2) Non-affine access where each gather index is relative to the
     initial address.

The assumption this patch works on is that once the alignment for the
first scalar is correct, all others will fall in line, as the index is
always a multiple of the first element's size.

For (1) we have a dataref and can check it for alignment as in other
cases.  For (2) this patch checks the object alignment of BASE and
compares it against the natural alignment of the current vectype's unit.

The patch also adds a pointer argument to the gather/scatter IFNs that
contains the necessary alignment.  Most of the patch is thus mechanical
in that it merely adjusts indices.

I tested the riscv version with a custom qemu version that faults on
element-misaligned vector accesses.  With this patch applied, there is
just a single fault left, which is due to PR120782 and which will be
addressed separately.

Bootstrapped and regtested on x86 and aarch64.  Regtested on
rv64gcv_zvl512b with and without unaligned vector support.

gcc/ChangeLog:

	* internal-fn.cc (internal_fn_len_index): Adjust indices for new
	alias_ptr param.
	(internal_fn_else_index): Ditto.
	(internal_fn_mask_index): Ditto.
	(internal_fn_stored_value_index): Ditto.
	(internal_fn_alias_ptr_index): Ditto.
	(internal_fn_offset_index): Ditto.
	(internal_fn_scale_index): Ditto.
	(internal_gather_scatter_fn_supported_p): Ditto.
	* internal-fn.h (internal_fn_alias_ptr_index): Ditto.
	* optabs-query.cc (supports_vec_gather_load_p): Ditto.
	* tree-vect-data-refs.cc (vect_check_gather_scatter): Add alias
	pointer.
	* tree-vect-patterns.cc (vect_recog_gather_scatter_pattern): Add
	alias pointer.
	* tree-vect-slp.cc (vect_get_operand_map): Adjust for alias
	pointer.
	* tree-vect-stmts.cc (vect_truncate_gather_scatter_offset): Add
	alias pointer and misalignment handling.
	(get_load_store_type): Move from here...
	(get_group_load_store_type): ...To here.
	(vectorizable_store): Add alias pointer.
	(vectorizable_load): Ditto.
	* tree-vectorizer.h (struct gather_scatter_info): Ditto.