Tighten up an internal LSR API that should check for NULL.

No test case, but should fix a scan_build warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177374 91177308-0d34-0410-b5e6-96231b3b80d8
Cleanup some SCEV logic a bit.

Make the code more obvious to scan-build and humans.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177375 91177308-0d34-0410-b5e6-96231b3b80d8