Match: Support form 2 for scalar signed integer .SAT_ADD

This patch would like to support the form 2 of the scalar signed
integer .SAT_ADD.  Aka below example:

Form 2:
  #define DEF_SAT_S_ADD_FMT_2(T, UT, MIN, MAX) \
  T __attribute__((noinline))              \
  sat_s_add_##T##_fmt_2 (T x, T y)         \
  {                                        \
    T sum = (UT)x + (UT)y;                 \
                                           \
    if ((x ^ y) < 0 || (sum ^ x) >= 0)     \
      return sum;                          \
                                           \
    return x < 0 ? MIN : MAX;              \
  }

DEF_SAT_S_ADD_FMT_2(int8_t, uint8_t, INT8_MIN, INT8_MAX)

We can tell the difference before and after this patch if backend
implemented the ssadd<m>3 pattern similar as below.

Before this patch:
   4   │ __attribute__((noinline))
   5   │ int8_t sat_s_add_int8_t_fmt_2 (int8_t x, int8_t y)
   6   │ {
   7   │   int8_t sum;
   8   │   unsigned char x.0_1;
   9   │   unsigned char y.1_2;
  10   │   unsigned char _3;
  11   │   signed char _4;
  12   │   signed char _5;
  13   │   int8_t _6;
  14   │   _Bool _11;
  15   │   signed char _12;
  16   │   signed char _13;
  17   │   signed char _14;
  18   │   signed char _22;
  19   │   signed char _23;
  20   │
  21   │ ;;   basic block 2, loop depth 0
  22   │ ;;    pred:       ENTRY
  23   │   x.0_1 = (unsigned char) x_7(D);
  24   │   y.1_2 = (unsigned char) y_8(D);
  25   │   _3 = x.0_1 + y.1_2;
  26   │   sum_9 = (int8_t) _3;
  27   │   _4 = x_7(D) ^ y_8(D);
  28   │   _5 = x_7(D) ^ sum_9;
  29   │   _23 = ~_4;
  30   │   _22 = _5 & _23;
  31   │   if (_22 >= 0)
  32   │     goto <bb 4>; [42.57%]
  33   │   else
  34   │     goto <bb 3>; [57.43%]
  35   │ ;;    succ:       4
  36   │ ;;                3
  37   │
  38   │ ;;   basic block 3, loop depth 0
  39   │ ;;    pred:       2
  40   │   _11 = x_7(D) < 0;
  41   │   _12 = (signed char) _11;
  42   │   _13 = -_12;
  43   │   _14 = _13 ^ 127;
  44   │ ;;    succ:       4
  45   │
  46   │ ;;   basic block 4, loop depth 0
  47   │ ;;    pred:       2
  48   │ ;;                3
  49   │   # _6 = PHI <sum_9(2), _14(3)>
  50   │   return _6;
  51   │ ;;    succ:       EXIT
  52   │
  53   │ }

After this patch:
   4   │ __attribute__((noinline))
   5   │ int8_t sat_s_add_int8_t_fmt_2 (int8_t x, int8_t y)
   6   │ {
   7   │   int8_t _6;
   8   │
   9   │ ;;   basic block 2, loop depth 0
  10   │ ;;    pred:       ENTRY
  11   │   _6 = .SAT_ADD (x_7(D), y_8(D)); [tail call]
  12   │   return _6;
  13   │ ;;    succ:       EXIT
  14   │
  15   │ }

The below test suites are passed for this patch.
* The rv64gcv fully regression test.
* The x86 bootstrap test.
* The x86 fully regression test.

gcc/ChangeLog:

	* match.pd: Add the form 2 of signed .SAT_ADD matching.

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