reassoc: Improve maybe_optimize_range_tests [PR96480]

reassoc: Improve maybe_optimize_range_tests [PR96480]

On the following testcase, if the IL before reassoc would be:
...
  <bb 4> [local count: 354334800]:
  if (x_3(D) == 2)
    goto <bb 7>; [34.00%]
  else
    goto <bb 5>; [66.00%]

  <bb 5> [local count: 233860967]:
  if (x_3(D) == 3)
    goto <bb 7>; [34.00%]
  else
    goto <bb 6>; [66.00%]

  <bb 6> [local count: 79512730]:

  <bb 7> [local count: 1073741824]:
  # prephitmp_7 = PHI <1(3), 1(4), 1(5), 1(2), 0(6)>
then we'd optimize it properly, but as bb 5-7 is instead:
  <bb 5> [local count: 233860967]:
  if (x_3(D) == 3)
    goto <bb 6>; [34.00%]
  else
    goto <bb 7>; [66.00%]

  <bb 6> [local count: 79512730]:

  <bb 7> [local count: 1073741824]:
  # prephitmp_7 = PHI <1(3), 1(4), 0(5), 1(2), 1(6)>
(i.e. the true/false edges on the last bb with condition swapped
and ditto for the phi args), we don't recognize it.  If bb 6
is empty, there should be no functional difference between the two IL
representations.

This patch handles those special cases.

2020-08-06  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96480
	* tree-ssa-reassoc.c (suitable_cond_bb): Add TEST_SWAPPED_P argument.
	If TEST_BB ends in cond and has one edge to *OTHER_BB and another
	through an empty bb to that block too, if PHI args don't match, retry
	them through the other path from TEST_BB.
	(maybe_optimize_range_tests): Adjust callers.  Handle such LAST_BB
	through inversion of the condition.

	* gcc.dg/tree-ssa/pr96480.c: New test.