Previously all code stubs (i.e. both platform and Crankshaft code stubs)
preserved the context register for full-codegen (neither Ignition, nor
TurboFan nor Crankshaft require this or would benefit from this), but
the newly introduced TurboFanCodeStubs no longer do this and there's no
need to, so we have to make sure in full-codegen that we restore the
context register after intrinsic calls, which potentially call
TurboFanCodeStubs.

Drive-by-fix: VisitThisFunction can be made platform independent.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/1848553002

Cr-Commit-Position: refs/heads/master@{#35154}