[turbofan] Use Start as sentinel for frame states.

[turbofan] Use Start as sentinel for frame states.

This simplifies inlining, in that we only need to update uses of Start
and inputs of End instead of walking the whole inlinee to update all
outer frame states.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1146403008

Cr-Commit-Position: refs/heads/master@{#28649}