[x86] Improve code generation for context materialization.

[x86] Improve code generation for context materialization.

On Intel targets, it is cheaper to load the context from the frame
instead of loading the context as a constant (which usually involves a
PropertyCell because the context is in new space when we compile the
function).

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/970803002

Cr-Commit-Position: refs/heads/master@{#26935}