Widening-Mul: Support unsigned scalar SAT_MUL form 1

Widening-Mul: Support unsigned scalar SAT_MUL form 1

This patch would like to try to match the SAT_MUL during
widening-mul pass, aka below pattern.

  NT __attribute__((noinline))
  sat_u_mul_##NT##_fmt_1 (NT a, NT b)
  {
    uint128_t x = (uint128_t)a * (uint128_t)b;
    NT max = -1;
    if (x > (uint128_t)(max))
      return max;
    else
      return (NT)x;
  }

while the NT can be uint8_t, uint16_t, uint32_t and uint64_t.

gcc/ChangeLog:

	* match.pd: Add new match pattern for unsigned SAT_MUL.
	* tree-ssa-math-opts.cc (gimple_unsigned_integer_sat_mul):
	new decl for pattern match func.
	(match_unsigned_saturation_mul): Add new func to match unsigned
	SAT_MUL.
	(math_opts_dom_walker::after_dom_children): Try to match
	unsigned SAT_MUL on NOP.

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