[Inliner][NFC] Remove redundant nullptr check

Remove the null pointer check on Callee since it is guaranteed to pass by the check
at the top of the loop which continues if Callee is null. While this change is somewhat
trivial, for what it's worth this check triggers Coverity warnings because it implies that
Callee might be null at this point even though it is dereferenced in the preceding code.

Reviewed By: mtrofin

Differential Revision: https://reviews.llvm.org/D145463