[SCEV] Consolidate comments; NFC

Consolidate documentation by removing comments from the .cpp file where
the comments in the .cpp file were copy-pasted from the header.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271157 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed