RISC-V: Optimize SELECT_VL codegen when length is known as smaller than VF

While trying to fix bugs of PR113097, notice this following situation we
generate redundant vsetvli

_255 = SELECT_VL (3, POLY_INT_CST [4, 4]);
COND_LEN (..., _255)

Before this patch:

vsetivli a5, 3...
...
vadd.vv (use a5)

After this patch:

...
vadd.vv (use AVL = 3)

The reason we can do this is because known_ge (3, [4,4]) is true.
It's safe to apply such optimization

Tested on both RV32 and RV64 full coverage testing, no regression.

	PR target/113087

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_select_vl): Optimize SELECT_VL.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr113087-2.c: New test.