RISC-V: Disable AVL propagation of slidedown instructions

RISC-V: Disable AVL propagation of slidedown instructions

Re-check again RVV ISA, I find that we can't allow AVL propagation not only
for vrgather, but also slidedown instructions.

Committed.

	PR target/112599

gcc/ChangeLog:

	* config/riscv/riscv-avlprop.cc (avl_can_be_propagated_p): Add slidedown.
	(vlmax_ta_p): Ditto.
	(pass_avlprop::get_vlmax_ta_preferred_avl): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/vf_avl-1.c: Adapt test.
	* gcc.target/riscv/rvv/autovec/pr112599-3.c: New test.