[turbofan] Reduce context accesses during typed lowering.

TEST=unittests
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/776243002

Cr-Commit-Position: refs/heads/master@{#25656}
Do not embed function in DirectLoadGlobalFunctionPrototype.

BUG=v8:3629
LOG=N

Review URL: https://codereview.chromium.org/778673002

Cr-Commit-Position: refs/heads/master@{#25657}