* toplev.c (warn_disabled_optimization): Declare new warning flag.
        * flags.h (warn_disabled_optimization): Add it here.
        * gcse.c (gcse_main): Add warning when disabled.
        * invoke.texi: Document -Wdisabled-optimization

Co-Authored-By: Mark Mitchell <mark@codesourcery.com>

From-SVN: r36568