revert: re PR lto/66752 (spec2000 255.vortex performance compiled with GCC is ~20% lower than with CLANG)

Revert:
        PR lto/66752
        * tree-ssa-threadedge.c (simplify_conrol_stmt_condition): If we are
        unable to find X NE 0 in the tables, return X as the simplified
        condition.
        (fsm_find_control_statement_thread_paths): If nodes in NEXT_PATH are
        in VISISTED_BBS, then return failure.  Else add nodes from NEXT_PATH
        to VISISTED_BBS.  */
        * tree-ssa-threadupdate.c (duplicate_thread_path): Fix up edge flags
        after removing the control flow statement and unnecessary edges.

    testsuite/
        PR lto/66752
        * gcc.dg/tree-ssa/pr66752-2.c: New test.
        * gcc.dg/torture/pr66752-1.c: New test
        * g++.dg/torture/pr66752-2.C: New test.

From-SVN: r226206