RISC-V: Implement scalar SAT_SUB for signed integer

RISC-V: Implement scalar SAT_SUB for signed integer

This patch would like to implement the sssub form 1.  Aka:

Form 1:
  #define DEF_SAT_S_SUB_FMT_1(T, UT, MIN, MAX) \
  T __attribute__((noinline))                  \
  sat_s_sub_##T##_fmt_1 (T x, T y)             \
  {                                            \
    T minus = (UT)x - (UT)y;                   \
    return (x ^ y) >= 0                        \
      ? minus                                  \
      : (minus ^ x) >= 0                       \
        ? minus                                \
        : x < 0 ? MIN : MAX;                   \
  }

DEF_SAT_S_SUB_FMT_1(int8_t, uint8_t, INT8_MIN, INT8_MAX)

Before this patch:
  10   │ sat_s_sub_int8_t_fmt_1:
  11   │     subw    a5,a0,a1
  12   │     slliw   a5,a5,24
  13   │     sraiw   a5,a5,24
  14   │     xor a1,a0,a1
  15   │     xor a4,a0,a5
  16   │     and a1,a1,a4
  17   │     blt a1,zero,.L4
  18   │     mv  a0,a5
  19   │     ret
  20   │ .L4:
  21   │     srai    a0,a0,63
  22   │     xori    a5,a0,127
  23   │     mv  a0,a5
  24   │     ret

After this patch:
  10   │ sat_s_sub_int8_t_fmt_1:
  11   │     sub a4,a0,a1
  12   │     xor a5,a0,a4
  13   │     xor a1,a0,a1
  14   │     and a5,a5,a1
  15   │     srli    a5,a5,7
  16   │     andi    a5,a5,1
  17   │     srai    a0,a0,63
  18   │     xori    a3,a0,127
  19   │     neg a0,a5
  20   │     addi    a5,a5,-1
  21   │     and a3,a3,a0
  22   │     and a0,a4,a5
  23   │     or  a0,a0,a3
  24   │     slliw   a0,a0,24
  25   │     sraiw   a0,a0,24
  26   │     ret

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

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_expand_sssub): Add new func
	decl for expanding signed SAT_SUB.
	* config/riscv/riscv.cc (riscv_expand_sssub): Add new func impl
	for expanding signed SAT_SUB.
	* config/riscv/riscv.md (sssub<mode>3): Add new pattern sssub
	for scalar signed integer.

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