[ProfileSummary] Refactor getFromMD to prepare for another optional field. NFC.

Summary:
Rename 'i' to 'I'.
Factor out the optional field handling to getOptionalVal().
Split out of D79951.

Reviewers: davidxl

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80230