RISCV: Fix PR111074 [GCC13 BUG]

his patch fixes this issue happens on GCC-13.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111074

This patch should be backported to GCC-13.
GCC-14 has rewritten propagate_avl function, so there is no issue.

PR target/111074

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (extract_single_source): Fix bug.