Split allocas in the entry block into two categories.  The first has alignment <= stack alignment and constant size.  The second violates one or both of those conditions.  Sort both of these lists in descending alignment order and emit.  Also, compute the need for a frame pointer during the pass.

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

Review URL: https://codereview.chromium.org/1414343010 .