[SCEV] Do not bother creating separate SCEVUnknown for unreachable nodes

Currently, SCEV creates SCEVUnknown for every node of unreachable code. If we
have a huge amounts of such code, we will be littering SE with these nodes. We could
just state that they all are undef and save some memory.

Differential Revision: https://reviews.llvm.org/D57567
Reviewed By: sanjoy


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353017 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed