veclower: improve selection of vector mode when lowering [PR 112787]

veclower: improve selection of vector mode when lowering [PR 112787]

This patch addresses the issue reported in PR target/112787 by improving the
compute type selection.  We do this by not considering types with more elements
than the type we are lowering since we'd reject such types anyway.

gcc/ChangeLog:

	PR target/112787
	* tree-vect-generic.cc (type_for_widest_vector_mode): Change function to
	use original vector type and check widest vector mode has at most the
	same number of elements.
	(get_compute_type): Pass original vector type rather than the element
	type to type_for_widest_vector_mode and remove now obsolete check for
	the number of elements.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr112787.c: New test.