RISC-V: Fix vec_series expander[PR110985]

This patch fix bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110985

gcc/ChangeLog:
	PR target/110985
	* config/riscv/riscv-v.cc (expand_vec_series): Refactor the expander.

gcc/testsuite/ChangeLog:
	PR target/110985
	* gcc.target/riscv/rvv/autovec/vls-vlmax/pr110985.c: New test.