fold-const.c (fold, [...]): When folding VAR++ == CONST or VAR-- == CONST construct a proper mask if...

        * fold-const.c (fold, case EQ_EXPR): When folding VAR++ == CONST
        or VAR-- == CONST construct a proper mask if VAR is a bitfield.
        Cope with CONST being out of range for the bitfield.

From-SVN: r20198