[turbofan] Context specialization should only specialize loads/stores.

[turbofan] Context specialization should only specialize loads/stores.

The JSContextSpecialization should only care about loads from the
context and stores to the context, where the context is either a
HeapConstant or the special context Parameter (and a context for the
outer most function is provided). This way we don't eagerly embed
arbitrary context constants for no benefit, but we still specialize the
loads and store which we actually care about.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1227963005

Cr-Commit-Position: refs/heads/master@{#29602}