tree-optimization/116352 - amend previous fix

tree-optimization/116352 - amend previous fix

The previous fix restricted external vector builds to defs from
the same basic-block.  That turns out too restrictive so we have
to mitigate the original issue in a different way which is
restricting it to the original case where all defs are in the
same basic-block.

	PR tree-optimization/116352
	* tree-vect-slp.cc (vect_build_slp_tree_2): When compressing
	operands from a two-operator node make sure the resulting
	operation does not mix defs from different basic-blocks.

(cherry picked from commit 1e8bd720b1a618a39e2a41eec05e935c32d295f3)