RISC-V: Enable len_mask{load, store} and remove len_{load, store}

RISC-V: Enable len_mask{load, store} and remove len_{load, store}

This patch enable len_mask_{load,store} to support flow-control in RVV auto-vectorization.

Consider this following case:
void
f (int32_t *__restrict a,
   int32_t *__restrict b,
   int32_t *__restrict cond,
   int n)
{
  for (int i = 0; i < n; i++)
    if (cond[i])
      a[i] = b[i];
}

Before this patch:
<source>:9:21: missed: couldn't vectorize loop
<source>:9:21: missed: not vectorized: control flow in loop.

After this patch:
f:
        ble     a3,zero,.L5
.L3:
        vsetvli a5,a3,e32,m1,ta,ma
        vle32.v v0,0(a2)
        vsetvli a6,zero,e32,m1,ta,ma
        slli    a4,a5,2
        vmsne.vi        v0,v0,0
        sub     a3,a3,a5
        vsetvli zero,a5,e32,m1,ta,ma
        vle32.v v1,0(a1),v0.t
        vse32.v v1,0(a0),v0.t
        add     a2,a2,a4
        add     a1,a1,a4
        add     a0,a0,a4
        bne     a3,zero,.L3
.L5:
        ret

gcc/ChangeLog:

	* config/riscv/autovec.md (len_load_<mode>): Remove.
	(len_maskload<mode><vm>): Remove.
	(len_store_<mode>): New pattern.
	(len_maskstore<mode><vm>): New pattern.
	* config/riscv/predicates.md (autovec_length_operand): New predicate.
	* config/riscv/riscv-protos.h (enum insn_type): New enum.
	(expand_load_store): New function.
	* config/riscv/riscv-v.cc (emit_vlmax_masked_insn): Ditto.
	(emit_nonvlmax_masked_insn): Ditto.
	(expand_load_store): Ditto.
	* config/riscv/riscv-vector-builtins.cc
	(function_expander::use_contiguous_store_insn): Add avl_type operand
	into pred_store.
	* config/riscv/vector.md: Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-2.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-2.h: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-3.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup-3.h: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup_run-2.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/single_rgroup_run-3.c: New test.