[DebugInfo] Add helper for finding entry value candidates [NFC]

Summary:
The conditions that are used to determine if entry values should be
emitted for a parameter are quite many, and will grow slightly
in a follow-up commit, so move those to a helper function, as was
suggested in the code review for D69889.

Reviewers: djtodoro, NikolaPrica

Reviewed By: djtodoro

Subscribers: probinson, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69955