[Attributor][NFC] Refactoring `AANoFreeArgument::updateImpl`

Summary: Refactoring `AANoFreeArgument::updateImpl`. There is no test change.

Reviewers: sstefan1, jdoerfert

Reviewed By: sstefan1

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71349