[SCEV] Add missing scPtrToAddr to getCastExpr. (#211899)
https://github.com/llvm/llvm-project/pull/180244 (8e76c3f0d29) missed
adding scPtrToAddr to getCastExpr, causing `Not a SCEV cast expression!`
on some of the AMDGPU bots, including
https://lab.llvm.org/buildbot/#/builders/10/builds/32555.
Add missing case to handle PtrToAddr.
