[Bug tree-optimization/81949] New: DOM fails to simplify conditional

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81949

            Bug ID: 81949
           Summary: DOM fails to simplify conditional
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at redhat dot com
  Target Milestone: ---

DOM fails to simplify the 3rd conditional in the test 20030922-2.c.  This is by
current design and the test is xfailed.

This is a regression relative to gcc-7 and earlier.

So solve this within the DOM framework we'd need to build unwindable
equivalence classes from the conditional equivalences.  Given those classes, we
could then iterate over the items within the equivalence class for each use
operand to see if using an equivalent SSA_NAME allows a given statement to
simplify (not just propagate, but truly simplify).