With this CL, context loads and stores are "strengthened" by reducing
the incoming context chain and decreasing the depth accordingly,
whenever possible.  This enables more opportunities for specialization
and will let us easily add module context specialization later.

BUG=

Review-Url: https://codereview.chromium.org/2559173003
Cr-Commit-Position: refs/heads/master@{#42334}