tree-if-conv.c: fix ICE seen with -fno-tree-forwprop (PR tree-optimization/84178)

tree-if-conv.c: fix ICE seen with -fno-tree-forwprop (PR tree-optimization/84178)

PR tree-optimization/84178 reports a couple of source files that ICE inside
ifcvt when compiled with -03 -fno-tree-forwprop (trunk and gcc 7).

Both cases involve problems with ifcvt's per-BB gimplified predicates.

Testcase 1 fails this assertion within release_bb_predicate during cleanup:

283	      if (flag_checking)
284		for (gimple_stmt_iterator i = gsi_start (stmts);
285		     !gsi_end_p (i); gsi_next (&i))
286		  gcc_assert (! gimple_use_ops (gsi_stmt (i)));

The testcase contains a division in the loop, which leads to
if_convertible_loop_p returning false (due to gimple_could_trap_p being true
for the division).  This happens *after* the per-BB gimplified predicates
have been created in predicate_bbs (loop).
Hence tree_if_conversion bails out to "cleanup", but the gimplified predicates
exist and make use of SSA names; for example this conjunction for two BB
conditions:

  _4 = h4.1_112 != 0;
  _175 = (signed char) _117;
  _176 = _175 >= 0;
  _174 = _4 & _176;

is using SSA names.

This assertion was added in r236498 (aka c3deca2519d97c55876869c57cf11ae1e5c6cf8b):

    2016-05-20  Richard Biener  <rguenther@suse.de>

        * tree-if-conv.c (add_bb_predicate_gimplified_stmts): Use
        gimple_seq_add_seq_without_update.
        (release_bb_predicate): Assert we have no operands to free.
        (if_convertible_loop_p_1): Calculate post dominators later.
        Do not free BB predicates here.
        (combine_blocks): Do not recompute BB predicates.
        (version_loop_for_if_conversion): Save BB predicates around
        loop versioning.

        * gcc.dg/tree-ssa/ifc-cd.c: Adjust.

The following patch fixes this by adding a call to gimple_seq_discard
to release_bb_predicate.  It also updates the assertion, so that
instead of asserting the stmts have no imm uses, instead assert that
they weren't added to a bb before discarding them (otherwise discarding
them would be a bug).  We know this is the case because
insert_gimplified_predicates has:

	  /* Once the sequence is code generated, set it to NULL.  */
	  set_bb_predicate_gimplified_stmts (bb, NULL);

but asserting it seems appropriate as a double-check.

The patch doesn't address the 2nd issue within PR tree-optimization/84178.

gcc/ChangeLog:
	PR tree-optimization/84178
	* tree-if-conv.c (release_bb_predicate): Remove the
	the assertion that the stmts have NULL use_ops.
	Discard the statements, asserting that they haven't
	yet been added to a BB.

gcc/testsuite/ChangeLog:
	PR tree-optimization/84178
	* gcc.c-torture/compile/pr84178-1.c: New test.

From-SVN: r258363