middle-end/100951 - make sure to generate VECTOR_CST in lowering

middle-end/100951 - make sure to generate VECTOR_CST in lowering

When vector lowering creates piecewise ops make sure to create
VECTOR_CSTs instead of CONSTRUCTORs when possible.

gcc/

2021-06-07  Richard Biener  <rguenther@suse.de>

	PR middle-end/100951
	* tree-vect-generic.c (expand_vector_piecewise): Build a
	VECTOR_CST if all elements are constant.
	(expand_vector_condition): Likewise.
	(lower_vec_perm): Likewise.
	(expand_vector_conversion): Likewise.

gcc/testsuite/

2021-06-07  H.J. Lu  <hjl.tools@gmail.com>

	PR middle-end/100951
	* gcc.target/i386/pr100951.c: New test.