1) Move helper creation to separate method, which also computes the maximum outgoing argument size.  The computed size is checked against call lowering's size.
2) Make addProlog use the outgoing argument size and remove the adjustments in lowerCall.
3) Remove AdjustStack instructions and friends.

BUG=
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1472623002 .