Internal-fn: Support new IFN SAT_ADD for unsigned scalar int

Internal-fn: Support new IFN SAT_ADD for unsigned scalar int

This patch would like to add the middle-end presentation for the
saturation add.  Aka set the result of add to the max when overflow.
It will take the pattern similar as below.

SAT_ADD (x, y) => (x + y) | (-(TYPE)((TYPE)(x + y) < x))

Take uint8_t as example, we will have:

* SAT_ADD (1, 254)   => 255.
* SAT_ADD (1, 255)   => 255.
* SAT_ADD (2, 255)   => 255.
* SAT_ADD (255, 255) => 255.

Given below example for the unsigned scalar integer uint64_t:

uint64_t sat_add_u64 (uint64_t x, uint64_t y)
{
  return (x + y) | (- (uint64_t)((uint64_t)(x + y) < x));
}

Before this patch:
uint64_t sat_add_uint64_t (uint64_t x, uint64_t y)
{
  long unsigned int _1;
  _Bool _2;
  long unsigned int _3;
  long unsigned int _4;
  uint64_t _7;
  long unsigned int _10;
  __complex__ long unsigned int _11;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _11 = .ADD_OVERFLOW (x_5(D), y_6(D));
  _1 = REALPART_EXPR <_11>;
  _10 = IMAGPART_EXPR <_11>;
  _2 = _10 != 0;
  _3 = (long unsigned int) _2;
  _4 = -_3;
  _7 = _1 | _4;
  return _7;
;;    succ:       EXIT

}

After this patch:
uint64_t sat_add_uint64_t (uint64_t x, uint64_t y)
{
  uint64_t _7;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _7 = .SAT_ADD (x_5(D), y_6(D)); [tail call]
  return _7;
;;    succ:       EXIT
}

The below tests are passed for this patch:
1. The riscv fully regression tests.
3. The x86 bootstrap tests.
4. The x86 fully regression tests.

	PR target/51492
	PR target/112600

gcc/ChangeLog:

	* internal-fn.cc (commutative_binary_fn_p): Add type IFN_SAT_ADD
	to the return true switch case(s).
	* internal-fn.def (SAT_ADD):  Add new signed optab SAT_ADD.
	* match.pd: Add unsigned SAT_ADD match(es).
	* optabs.def (OPTAB_NL): Remove fixed-point limitation for
	us/ssadd.
	* tree-ssa-math-opts.cc (gimple_unsigned_integer_sat_add): New
	extern func decl generated in match.pd match.
	(match_saturation_arith): New func impl to match the saturation arith.
	(math_opts_dom_walker::after_dom_children): Try match saturation
	arith when IOR expr.

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