[PR 47086] ICE: verify flow info failed

[Jeff Law <law at redhat dot com>]

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This is a case where we have a loop in which the counter is totally
unused, except to count.  Exit from the loop is triggered on an overflow
trap when the counter is incremented (yes, compiled with -ftrapv
- -fexceptions -fno-call-exceptions).

GCC issues a checking failure after IV optimizations because we have a
block with an EH edge, but IV opts have removed the IV increment which
was the only statement which could trap.

The fix is relatively simple, but a bit of a big hammer -- we just fail
to recognize a statement that map trap as creating a IV.

Thus the IV code won't delete the unused IV which keeps the CFG verifier
happy and also keep the loop from running infinitely.

Bootstrapped and regression tested on x86_64-pc-linux-gnu.

OK for the trunk?


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