RISC-V: Minor tweak dynamic cost model

v2 update: Robostify tests.

While working on cost model, I notice one case that dynamic lmul cost doesn't work well.

Before this patch:

foo:
        lui     a4,%hi(.LANCHOR0)
        li      a0,1953
        li      a1,63
        addi    a4,a4,%lo(.LANCHOR0)
        li      a3,64
        vsetvli a2,zero,e32,mf2,ta,ma
        vmv.v.x v5,a0
        vmv.v.x v4,a1
        vid.v   v3
.L2:
        vsetvli a5,a3,e32,mf2,ta,ma
        vadd.vi v2,v3,1
        vadd.vv v1,v3,v5
        mv      a2,a5
        vmacc.vv        v1,v2,v4
        slli    a1,a5,2
        vse32.v v1,0(a4)
        sub     a3,a3,a5
        add     a4,a4,a1
        vsetvli a5,zero,e32,mf2,ta,ma
        vmv.v.x v1,a2
        vadd.vv v3,v3,v1
        bne     a3,zero,.L2
        li      a0,0
        ret

Unexpected: Use scalable vector and LMUL = MF2 which is wasting computation resources.

Ideally, we should use LMUL = M8 VLS modes.

The root cause is the dynamic LMUL heuristic dominates the VLS heuristic.
Adapt the cost model heuristic.

After this patch:

foo:
	lui	a4,%hi(.LANCHOR0)
	addi	a4,a4,%lo(.LANCHOR0)
	li	a3,4096
	li	a5,32
	li	a1,2016
	addi	a2,a4,128
	addiw	a3,a3,-32
	vsetvli	zero,a5,e32,m8,ta,ma
	li	a0,0
	vid.v	v8
	vsll.vi	v8,v8,6
	vadd.vx	v16,v8,a1
	vadd.vx	v8,v8,a3
	vse32.v	v16,0(a4)
	vse32.v	v8,0(a2)
	ret

Tested on both RV32/RV64 no regression.

Ok for trunk ?

gcc/ChangeLog:

	* config/riscv/riscv-vector-costs.cc (costs::better_main_loop_than_p): Minior tweak.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-10.c: Fix test.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-11.c: Ditto.
	* gcc.dg/vect/costmodel/riscv/rvv/vla_vs_vls-12.c: Ditto.