testsuite: Update some vect cases for partial vectors

testsuite: Update some vect cases for partial vectors

This patch is to adjust some existing vectorization test cases
to work well with the newly introduced partial vector usages.

Bootstrapped/regtested on aarch64-linux-gnu and powerpc64le-linux-gnu
P9 (with explicit param vect-partial-vector-usage=1 and enablement on
check_effective_target_vect_partial_vectors_usage_1 check).

gcc/ChangeLog:

	* doc/sourcebuild.texi (vect_len_load_store,
	vect_partial_vectors_usage_1, vect_partial_vectors_usage_2,
	vect_partial_vectors): Document.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/bb-slp-pr69907.c: Adjust for partial vector usages.
	* gcc.dg/vect/slp-3.c: Likewise.
	* gcc.dg/vect/slp-multitypes-11.c: Likewise.
	* gcc.dg/vect/slp-perm-1.c: Likewise.
	* gcc.dg/vect/slp-perm-5.c: Likewise.
	* gcc.dg/vect/slp-perm-6.c: Likewise.
	* gcc.dg/vect/slp-perm-7.c: Likewise.
	* gcc.dg/vect/slp-perm-8.c: Likewise.
	* gcc.dg/vect/slp-perm-9.c: Likewise.
	* gcc.dg/vect/vect-version-2.c: Likewise.
	* lib/target-supports.exp (check_vect_partial_vector_usage): New
	function.
	(check_effective_target_vect_len_load_store): Likewise.
	(check_effective_target_vect_partial_vectors_usage_1): Likewise.
	(check_effective_target_vect_partial_vectors_usage_2): Likewise.
	(check_effective_target_vect_partial_vectors): Likewise.