PR111754: Rework encoding of result for VEC_PERM_EXPR with constant input vectors.

PR111754: Rework encoding of result for VEC_PERM_EXPR with constant input vectors.

gcc/ChangeLog:
	PR middle-end/111754
	* fold-const.cc (fold_vec_perm_cst): Set result's encoding to sel's
	encoding, and set res_nelts_per_pattern to 2 if sel contains stepped
	sequence but input vectors do not.
	(test_nunits_min_2): New test Case 8.
	(test_nunits_min_4): New tests Case 8 and Case 9.

gcc/testsuite/ChangeLog:
	PR middle-end/111754
	* gcc.target/aarch64/sve/slp_3.c: Adjust code-gen.
	* gcc.target/aarch64/sve/slp_4.c: Likewise.
	* gcc.dg/vect/pr111754.c: New test.

Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>