RISC-V: Set the natural size of constant vector mask modes to one RVV data vector.

RISC-V: Set the natural size of constant vector mask modes to one RVV data vector.

If reinterpret vnx2bi as vnx16qi, vnx16qi must occupy no more of the underlying
registers than vnx2bi.

Consider this following case:
void test_vreinterpret_v_b64_i8m1 (uint8_t *in, int8_t *out)
{
  vbool64_t vmask = __riscv_vlm_v_b64 (in, 2);
  vint8m1_t vout = __riscv_vreinterpret_v_b64_i8m1 (vmask);
  __riscv_vse8_v_i8m1(out, vout, 16);
}

compiler parameters: -march=rv64gcv -mabi=lp64d --param=riscv-autovec-preference=fixed-vlmax -O3
Compilation fails with:
test_vreinterpret_v_b64_i8m1during RTL pass: expand

test.c: In function 'test_vreinterpret_v_b64_i8m1':
test.c:11:22: internal compiler error: in gen_lowpart_general, at rtlhooks.cc:57
   11 |     vint8m1_t vout = __riscv_vreinterpret_v_b64_i8m1(src);
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0xf11876 gen_lowpart_general(machine_mode, rtx_def*)
        ../.././riscv-gcc/gcc/rtlhooks.cc:57
0x191435e gen_vreinterpretvnx16qi(rtx_def*, rtx_def*)
        ../.././riscv-gcc/gcc/config/riscv/vector.md:486
0xe08858 maybe_expand_insn(insn_code, unsigned int, expand_operand*)
        ../.././riscv-gcc/gcc/optabs.cc:8213
0x1471209 riscv_vector::function_expander::generate_insn(insn_code)
        ../.././riscv-gcc/gcc/config/riscv/riscv-vector-builtins.cc:3813
0x147629c riscv_vector::function_expander::expand()
        ../.././riscv-gcc/gcc/config/riscv/riscv-vector-builtins.h:520
0x147629c riscv_vector::expand_builtin(unsigned int, tree_node*, rtx_def*)
        ../.././riscv-gcc/gcc/config/riscv/riscv-vector-builtins.cc:4103
0x9868f9 expand_builtin(tree_node*, rtx_def*, rtx_def*, machine_mode, int)
        ../.././riscv-gcc/gcc/builtins.cc:7342

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_regmode_natural_size): set the natural
	size of vector mask mode to one rvv register.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vreinterpet-fixed.c: New test.