[turbofan] Use reference equal to zero instead of a smi check.

[turbofan] Use reference equal to zero instead of a smi check.

In typed lowering we can use the ReferenceEqual simplified operator
instead of ObjectIsSmi to check for context extensions. This generates
the desired code.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1163963003

Cr-Commit-Position: refs/heads/master@{#28776}