[llvm] [ThinLTO] Avoid creating map entries on lookup (NFCI) (PR #164873)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

> Hi @teresajohnson, I was looking into backporting #166067 to the 21.x release and was wondering if this change could also be backported?

Sure. Note I didn't measure any obvious improvement but I only looked at one application.

https://github.com/llvm/llvm-project/pull/164873