[llvm] [polly] [IR] Don't allow successors() over block without terminators (PR #186646)

[Alexis Engelke via llvm-commits]

aengelke wrote:

So the OpenMP does sketchy things also on other occasions and works around limitation by temporarily adding unreachable instructions. Can't hurt to do this three more times... same for Polly.

The c-t impact is [0.10% stage2-O3](http://llvm-compile-time-tracker.com/compare.php?from=27e4ef689a3aa93173dbd212de8aa5a2b5f65402&to=e200e9264d513d8ea967fd07fdc49593932e5fc3&stat=instructions:u) -- more than I expected.

https://github.com/llvm/llvm-project/pull/186646