Add Python checks to prevent mixing ops from different while loops.

The executor can currently catch some errors like this by trying to
reconstruct the while loop contexts by tracing the graph from enter
nodes, but this doesn't catch everything and can cause hangs or other
undesirable behavior. This change puts the check in Python and also
provides better debugging information.

In addition, this change refactors some logic from control_flow_ops.py
to a new file, control_flow_util.py. This is so we can call
CheckInputFromValidContext from ops.py without creating circular imports
between ops.py and control_flow_ops.py.

PiperOrigin-RevId: 178161679