RISC-V: Implement vector SAT_SUB for signed integer

RISC-V: Implement vector SAT_SUB for signed integer

This patch would like to implement the sssub for vector signed integer.

Form 1:
  #define DEF_VEC_SAT_S_SUB_FMT_1(T, UT, MIN, MAX)                     \
  void __attribute__((noinline))                                       \
  vec_sat_s_add_##T##_fmt_1 (T *out, T *op_1, T *op_2, unsigned limit) \
  {                                                                    \
    unsigned i;                                                        \
    for (i = 0; i < limit; i++)                                        \
      {                                                                \
        T x = op_1[i];                                                 \
        T y = op_2[i];                                                 \
        T minus = (UT)x - (UT)y;                                       \
        out[i] = (x ^ y) >= 0                                          \
          ? minus                                                      \
          : (minus ^ x) >= 0                                           \
            ? minus                                                    \
            : x < 0 ? MIN : MAX;                                       \
      }                                                                \
  }

DEF_VEC_SAT_S_SUB_FMT_1(int8_t, uint8_t, INT8_MIN, INT8_MAX)

Before this patch:
  28   │     vle8.v  v1,0(a1)
  29   │     vle8.v  v2,0(a2)
  30   │     sub a3,a3,a5
  31   │     add a1,a1,a5
  32   │     add a2,a2,a5
  33   │     vsra.vi v4,v1,7
  34   │     vsub.vv v3,v1,v2
  35   │     vxor.vv v2,v1,v2
  36   │     vxor.vv v0,v1,v3
  37   │     vmslt.vi    v2,v2,0
  38   │     vmslt.vi    v0,v0,0
  39   │     vmand.mm    v0,v0,v2
  40   │     vxor.vv v3,v4,v5,v0.t
  41   │     vse8.v  v3,0(a0)
  42   │     add a0,a0,a5

After this patch:
  25   │     vle8.v  v1,0(a1)
  26   │     vle8.v  v2,0(a2)
  27   │     sub a3,a3,a5
  28   │     add a1,a1,a5
  29   │     add a2,a2,a5
  30   │     vssub.vv    v1,v1,v2
  31   │     vse8.v  v1,0(a0)
  32   │     add a0,a0,a5

The below test suites are passed for this patch.
* The rv64gcv fully regression test.

gcc/ChangeLog:

	* config/riscv/autovec.md (sssub<mode>3): Add new pattern for
	signed SAT_SUB.
	* config/riscv/riscv-protos.h (expand_vec_sssub): Add new func
	decl to expand sssub to vssub.
	* config/riscv/riscv-v.cc (expand_vec_sssub): Add new func
	impl to expand sssub to vssub.

Signed-off-by: Pan Li <pan2.li@intel.com>