RISC-V: Adjust loop len by costing 1 when NITER < VF

RISC-V: Adjust loop len by costing 1 when NITER < VF

Rebase in v3: Rebase to the trunk and commit it as it's approved by Robin.
Update in v2: Add dynmaic lmul test.

This patch fixes the regression between GCC 13.2.0 and trunk GCC (GCC-14)

GCC 13.2.0:

	lui	a5,%hi(a)
	li	a4,19
	sb	a4,%lo(a)(a5)
	li	a0,0
	ret

Trunk GCC:

        vsetvli a5,zero,e8,mf2,ta,ma
        li      a4,-32768
        vid.v   v1
        vsetvli zero,zero,e16,m1,ta,ma
        addiw   a4,a4,104
        vmv.v.i v3,15
        lui     a1,%hi(a)
        li      a0,19
        vsetvli zero,zero,e8,mf2,ta,ma
        vadd.vi v1,v1,1
        sb      a0,%lo(a)(a1)
        vsetvli zero,zero,e16,m1,ta,ma
        vzext.vf2       v2,v1
        vmv.v.x v1,a4
        vminu.vv        v2,v2,v3
        vsrl.vv v1,v1,v2
        vslidedown.vi   v1,v1,17
        vmv.x.s a0,v1
        snez    a0,a0
        ret

The root cause we are vectorizing the codes inefficiently since we doesn't cost len when NITERS < VF.
Leverage loop control of mask targets or rs6000 fixes the regression.

Tested no regression. Ok for trunk ?

	PR target/113281

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (costs::adjust_vect_cost_per_loop): New function.
	(costs::finish_cost): Adjust cost for LOOP LEN with NITERS < VF.
	* config/riscv/riscv-vector-costs.h: New function.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/pr113281-3.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/pr113281-4.c: New test.
	* gcc.dg/vect/costmodel/riscv/rvv/pr113281-5.c: New test.