vect: Handle grouped accesses via gather/scatter.

vect: Handle grouped accesses via gather/scatter.

This patch adds gather/scatter handling for grouped access.  The idea is
to e.g. replace an access (for uint8_t elements) like
  arr[0]
  arr[1]
  arr[2]
  arr[3]
  arr[0 + step]
  arr[1 + step]
  ...
by gather loads of uint32_t
  arr[0..3]
  arr[0 + step * 1..3 + step * 1]
  arr[0 + step * 2..3 + step * 2]
  ...
where the offset vector is a simple series with step STEP.
If supported, such a gather can be implemented as a strided load.

If we have a masked access the transformation is not performed.
Masking could still be done after converting the data back to the
original vectype but it does not seem worth it for now.

	PR target/118019

gcc/ChangeLog:

	* internal-fn.cc (get_supported_else_vals): Exit at invalid
	index.
	(internal_strided_fn_supported_p): New funtion.
	* internal-fn.h (internal_strided_fn_supported_p): Declare.
	* tree-vect-stmts.cc (vector_vector_composition_type):
	Add vector_only argument.
	(vect_use_grouped_gather): New function.
	(vect_get_store_rhs): Adjust docs of
	vector_vector_composition_type.
	(get_load_store_type): Try grouped gather.
	(vectorizable_store): Use punned vectype.
	(vectorizable_load): Ditto.
	* tree-vectorizer.h (struct vect_load_store_data): Add punned
	vectype.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr118019-2.c: New test.