This patch optimizes the following seriese vector:
[nunits - 1, nunits - 2, ...., 0]

Before this patch:
vid
vmul
vadd

After this patch:
vid
vrsub

This patch is an obvious and simple optimization, ok for trunk?

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_vec_series): Optimize reverse series index vector.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls-vlmax/perm-4.c: Add assembly check.