This CL introduces a small struct to hold the {mem_start} and {mem_size}
node pointers that are managed in the function body decoder's SSA environment.
This struct insulates the function body decoder from further changes in
how context-specific information is represented in the compiler.

R=clemensh@chromium.org
CC=​mstarzinger@chromium.org

Bug: 
Change-Id: If17bef9fd2490ac11e4f3b3614f91333bb0b9528
Reviewed-on: https://chromium-review.googlesource.com/817282Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben L. Titzer <titzer@google.com>
Cr-Commit-Position: refs/heads/master@{#50004}