[ScopBuilder/Simplify] Refactor isEscaping. NFC.
ScopBuilder and Simplify (through VirtualInstruction.cpp) previously
used this functionality in their own implementation. Refactor them
both into a common one into the Scop class.
BlockGenerator also makes use of a similiar functionality, but also
records outside users and takes place after region simplification.
Merging it as well would be more complicated.
git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@309273 91177308-0d34-0410-b5e6-96231b3b80d8