middle-end: Support recognition of three-way max/min.

This patch adds support for three-way min/max recognition in phi-opts.

Concretely for e.g.

#include <stdint.h>

uint8_t three_min (uint8_t xc, uint8_t xm, uint8_t xy) {
	uint8_t	 xk;
    if (xc < xm) {
        xk = (uint8_t) (xc < xy ? xc : xy);
    } else {
        xk = (uint8_t) (xm < xy ? xm : xy);
    }
    return xk;
}

we generate:

  <bb 2> [local count: 1073741824]:
  _5 = MIN_EXPR <xc_1(D), xy_3(D)>;
  _7 = MIN_EXPR <xm_2(D), _5>;
  return _7;

instead of

  <bb 2>:
  if (xc_2(D) < xm_3(D))
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  xk_5 = MIN_EXPR <xc_2(D), xy_4(D)>;
  goto <bb 5>;

  <bb 4>:
  xk_6 = MIN_EXPR <xm_3(D), xy_4(D)>;

  <bb 5>:
  # xk_1 = PHI <xk_5(3), xk_6(4)>
  return xk_1;

The same function also immediately deals with turning a minimization problem
into a maximization one if the results are inverted.  We do this here since
doing it in match.pd would end up changing the shape of the BBs and adding
additional instructions which would prevent various optimizations from working.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (minmax_replacement): Optionally search for the phi
	sequence of a three-way conditional.
	(replace_phi_edge_with_variable): Support diamonds.
	(tree_ssa_phiopt_worker): Detect diamond phi structure for three-way
	min/max.
	(strip_bit_not, invert_minmax_code): New.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/split-path-1.c: Disable phi-opts so we don't optimize
	code away.
	* gcc.dg/tree-ssa/minmax-10.c: New test.
	* gcc.dg/tree-ssa/minmax-11.c: New test.
	* gcc.dg/tree-ssa/minmax-12.c: New test.
	* gcc.dg/tree-ssa/minmax-13.c: New test.
	* gcc.dg/tree-ssa/minmax-14.c: New test.
	* gcc.dg/tree-ssa/minmax-15.c: New test.
	* gcc.dg/tree-ssa/minmax-16.c: New test.
	* gcc.dg/tree-ssa/minmax-3.c: New test.
	* gcc.dg/tree-ssa/minmax-4.c: New test.
	* gcc.dg/tree-ssa/minmax-5.c: New test.
	* gcc.dg/tree-ssa/minmax-6.c: New test.
	* gcc.dg/tree-ssa/minmax-7.c: New test.
	* gcc.dg/tree-ssa/minmax-8.c: New test.
	* gcc.dg/tree-ssa/minmax-9.c: New test.