[RISC-V][PR target/120642] Avoid propagating constant AVL for theadvector

AVL propagation currently assumes that it can propagate a constant AVL into any
vector insn and trips an assert if the insn fails to recognize after such a
propagation.

However, for xtheadvector that is not a correct assumption; xtheadvector does
not allow the vector length to be a constant integer (other than zero which
allowed via x0).

After consulting with Jin Ma (thanks!) we agree the right fix is to avoid
creating the immediate AVL for xtheadvector.

This has been tested in my tester, just waiting for the pre-commit tester to
spin it.

	PR target/120642
gcc/
	* config/riscv/riscv-avlprop.cc (pass_avlprop::execute): Do not do
	constant AVL propagation for xtheadvector.

gcc/testsuite/
	* gcc.target/riscv/rvv/xtheadvector/pr120642.c: New test.

(cherry picked from commit f2e3886a30c771b104bc2714992e072b21a52e76)