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

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

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

SAT_SUB (x, y) => (x - y) & (-(TYPE)(x >= y));

For example for uint8_t, we have

* SAT_SUB (255, 0)   => 255
* SAT_SUB (1, 2)     => 0
* SAT_SUB (254, 255) => 0
* SAT_SUB (0, 255)   => 0

Given below SAT_SUB for uint64

uint64_t sat_sub_u64 (uint64_t x, uint64_t y)
{
  return (x - y) & (-(TYPE)(x >= y));
}

Before this patch:
uint64_t sat_sub_u_0_uint64_t (uint64_t x, uint64_t y)
{
  _Bool _1;
  long unsigned int _3;
  uint64_t _6;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _1 = x_4(D) >= y_5(D);
  _3 = x_4(D) - y_5(D);
  _6 = _1 ? _3 : 0;
  return _6;
;;    succ:       EXIT
}

After this patch:
uint64_t sat_sub_u_0_uint64_t (uint64_t x, uint64_t y)
{
  uint64_t _6;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _6 = .SAT_SUB (x_4(D), y_5(D)); [tail call]
  return _6;
;;    succ:       EXIT
}

The below tests are running for this patch:
*. The riscv fully regression tests.
*. The x86 bootstrap tests.
*. The x86 fully regression tests.

	PR target/51492
	PR target/112600

gcc/ChangeLog:

	* internal-fn.def (SAT_SUB): Add new IFN define for SAT_SUB.
	* match.pd: Add new match for SAT_SUB.
	* optabs.def (OPTAB_NL): Remove fixed-point for ussub/ssub.
	* tree-ssa-math-opts.cc (gimple_unsigned_integer_sat_sub): Add
	new decl for generated in match.pd.
	(build_saturation_binary_arith_call): Add new helper function
	to build the gimple call to binary SAT alu.
	(match_saturation_arith): Rename from.
	(match_unsigned_saturation_add): Rename to.
	(match_unsigned_saturation_sub): Add new func to match the
	unsigned sat sub.
	(math_opts_dom_walker::after_dom_children): Add SAT_SUB matching
	try when COND_EXPR.

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