p4c-bm2-ss crash when control instantiated but not apply'd

[jafingerhut]

In the attached P4_16 program, search for comment containing XXX. A control block instantiates an earlier control block. If it also calls apply() method on it, then p4c-bm2-ss compiles without errors.

If it does not also call apply() method on it, p4c-bm2-ss crashes.
crash2.p4.txt

[mihaibudiu]

The compiler front-end used to delete such dead code, but now it's leaving it in the program.
This causes a crash since the back-end does not expect them - all used instantiations should be inlined.
We are leaving instantiations in the program until the back-end even if they are unused, because they may have a bearing on the control-plane.
Note that the front-end still deletes unused tables; maybe we should change this fact as well.
I will submit a PR which fixes this crash.

[jafingerhut]

I have tested the P4 program crash2.p4 causing the crash before this fix was committed: 52c5e1c

No crash after the commit, neither with p4test nor p4c-bm2-ss -- only a correct warning about an unused instance. Closing this issue.