[Statepoints] Update DAG root after emitting statepoint.

Since we always generate CopyToRegs for statepoint results,
we must update DAG root after emitting statepoint, so that
these copies are scheduled before any possible local uses.
Note: getControlRoot() flushes all PendingExports, not only
those we generates for relocates. If that'll become a problem,
we can change it to flushing relocate exports only.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D87251