* common.opt: Add -fcombine-stack-adjustments.
        * opts.c (decode_options): Enable -fcombine-stack-adjustments at
        -O1.
        * combine-stack-adj.c (gate_handle_stack_adjustments): Check
        flag_combine_stack_adjustments.
        * doc/invoke.texi: Document the new option.

From-SVN: r164883