Match: Support form 1 for scalar signed integer .SAT_ADD

This patch would like to support the form 1 of the scalar signed
integer .SAT_ADD.  Aka below example:

Form 1:
  #define DEF_SAT_S_ADD_FMT_1(T, UT, MIN, MAX) \
  T __attribute__((noinline))                  \
  sat_s_add_##T##_fmt_1 (T x, T y)             \
  {                                            \
    T sum = (UT)x + (UT)y;                     \
    return (x ^ y) < 0                         \
      ? sum                                    \
      : (sum ^ x) >= 0                         \
        ? sum                                  \
        : x < 0 ? MIN : MAX;                   \
  }

DEF_SAT_S_ADD_FMT_1(int64_t, uint64_t, INT64_MIN, INT64_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   │ int64_t sat_s_add_int64_t_fmt_1 (int64_t x, int64_t y)
   6   │ {
   7   │   int64_t sum;
   8   │   long unsigned int x.0_1;
   9   │   long unsigned int y.1_2;
  10   │   long unsigned int _3;
  11   │   long int _4;
  12   │   long int _5;
  13   │   int64_t _6;
  14   │   _Bool _11;
  15   │   long int _12;
  16   │   long int _13;
  17   │   long int _14;
  18   │   long int _16;
  19   │   long int _17;
  20   │
  21   │ ;;   basic block 2, loop depth 0
  22   │ ;;    pred:       ENTRY
  23   │   x.0_1 = (long unsigned int) x_7(D);
  24   │   y.1_2 = (long unsigned int) y_8(D);
  25   │   _3 = x.0_1 + y.1_2;
  26   │   sum_9 = (int64_t) _3;
  27   │   _4 = x_7(D) ^ y_8(D);
  28   │   _5 = x_7(D) ^ sum_9;
  29   │   _17 = ~_4;
  30   │   _16 = _5 & _17;
  31   │   if (_16 < 0)
  32   │     goto <bb 3>; [41.00%]
  33   │   else
  34   │     goto <bb 4>; [59.00%]
  35   │ ;;    succ:       3
  36   │ ;;                4
  37   │
  38   │ ;;   basic block 3, loop depth 0
  39   │ ;;    pred:       2
  40   │   _11 = x_7(D) < 0;
  41   │   _12 = (long int) _11;
  42   │   _13 = -_12;
  43   │   _14 = _13 ^ 9223372036854775807;
  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   │ int64_t sat_s_add_int64_t_fmt_1 (int64_t x, int64_t y)
   6   │ {
   7   │   int64_t _4;
   8   │
   9   │ ;;   basic block 2, loop depth 0
  10   │ ;;    pred:       ENTRY
  11   │   _4 = .SAT_ADD (x_5(D), y_6(D)); [tail call]
  12   │   return _4;
  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 matching for signed .SAT_ADD.
	* tree-ssa-math-opts.cc (gimple_signed_integer_sat_add): Add new
	matching func decl.
	(match_unsigned_saturation_add): Try signed .SAT_ADD and rename
	to ...
	(match_saturation_add): ... here.
	(math_opts_dom_walker::after_dom_children): Update the above renamed
	func from caller.

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