Refactor isPointerOffset (NFC).

Summary:
Simplify the API using Optional<> and address comments in
         https://reviews.llvm.org/D66165

Reviewers: vitalybuka

Subscribers: hiraditya, llvm-commits, ostannard, pcc

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66317

llvm-svn: 369300