The JSInliner used to load the context from the JSFunction node at
runtime, which introduced a HeapConstant (because we had to materialize
the JSFunction after context specialization) and a LoadField operation,
independent whether the inlinee actually uses the context. This is
rather cumbersome currently, and therefore this is now changed to just
embed the context constant instead. Once we do inlining based on
SharedFunctionInfo rather than JSFunction, we should reconsider this
decision and come up with a proper heuristic.

BUG=v8:3952
LOG=n
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/994523002

Cr-Commit-Position: refs/heads/master@{#27069}