Widening-Mul: Support unsigned scalar SAT_MUL 2

Widening-Mul: Support unsigned scalar SAT_MUL 2

For mul_overflow api, we will have PHI node similar as below:

  _6 = .MUL_OVERFLOW (a_4(D), b_5(D));
  _2 = IMAGPART_EXPR <_6>;
  if (_2 != 0)
    goto <bb 4>; [35.00%]
  else
    goto <bb 3>; [65.00%]

  <bb 3> [local count: 697932184]:
  _1 = REALPART_EXPR <_6>;

  <bb 4> [local count: 1073741824]:
  # _3 = PHI <18446744073709551615(2), _1(3)>

Based on the help of match.pd pattern match, the widening-mul will
try to convert it to below:

  _3 = .SAT_MUL (a_4(D), b_5(D));

gcc/ChangeLog:

	* tree-ssa-math-opts.cc (match_saturation_mul): Add new func
	to emit IFN_SAT_MUL if matched.
	(math_opts_dom_walker::after_dom_children): Try to match
	the phi node for SAT_MUL.

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