RISC-V: Add simplification of dummy len and dummy mask COND_LEN_xxx pattern

In https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=d1eacedc6d9ba9f5522f2c8d49ccfdf7939ad72d
I optimize COND_LEN_xxx pattern with dummy len and dummy mask with too simply solution which
causes redundant vsetvli in the following case:

	vsetvli	a5,a2,e8,m1,ta,ma
	vle32.v	v8,0(a0)
	vsetivli	zero,16,e32,m4,tu,mu   ----> We should apply VLMAX instead of a CONST_INT AVL
	slli	a4,a5,2
	vand.vv	v0,v8,v16
	vand.vv	v4,v8,v12
	vmseq.vi	v0,v0,0
	sub	a2,a2,a5
	vneg.v	v4,v8,v0.t
	vsetvli	zero,a5,e32,m4,ta,ma

The root cause above is the following codes:

is_vlmax_len_p (...)
   return poly_int_rtx_p (len, &value)
        && known_eq (value, GET_MODE_NUNITS (mode))
        && !satisfies_constraint_K (len);            ---> incorrect check.

Actually, we should not elide the VLMAX situation that has AVL in range of [0,31].

After removing the the check above, we will have this following issue:

        vsetivli        zero,4,e32,m1,ta,ma
        vlseg4e32.v     v4,(a5)
        vlseg4e32.v     v12,(a3)
        vsetvli a5,zero,e32,m1,tu,ma             ---> This is redundant since VLMAX AVL = 4 when it is fixed-vlmax
        vfadd.vf        v3,v13,fa0
        vfadd.vf        v1,v12,fa1
        vfmul.vv        v17,v3,v5
        vfmul.vv        v16,v1,v5

Since all the following operations (vfadd.vf ... etc) are COND_LEN_xxx with dummy len and dummy mask,
we add the simplification operations dummy len and dummy mask into VLMAX TA and MA policy.

So, after this patch. Both cases are optimal codegen now:

case 1:
	vsetvli	a5,a2,e32,m1,ta,mu
	vle32.v	v2,0(a0)
	slli	a4,a5,2
	vand.vv	v1,v2,v3
	vand.vv	v0,v2,v4
	sub	a2,a2,a5
	vmseq.vi	v0,v0,0
	vneg.v	v1,v2,v0.t
	vse32.v	v1,0(a1)

case 2:
	vsetivli zero,4,e32,m1,tu,ma
	addi a4,a5,400
	vlseg4e32.v v12,(a3)
	vfadd.vf v3,v13,fa0
	vfadd.vf v1,v12,fa1
	vlseg4e32.v v4,(a4)
	vfadd.vf v2,v14,fa1
	vfmul.vv v17,v3,v5
	vfmul.vv v16,v1,v5

This patch is just additional fix of previous approved patch.
Tested on both RV32 and RV64 newlib no regression. Committed.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (is_vlmax_len_p): Remove satisfies_constraint_K.
	(expand_cond_len_op): Add simplification of dummy len and dummy mask.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/vf_avl-3.c: New test.