[llvm] [ThinLTO] Simplify checking for single external copy (NFCI) (PR #164861)

[Mingming Liu via llvm-commits]

https://github.com/mingmingl-llvm approved this pull request.

LGTM since compiler won't internalize more by reading the diff itself.

If I understand correctly, the surrounding comment explains why `ExternallyVisibleCopies` before and  `SingleExternallyVisibleCopy` are equivalent, and this change won't cause compiler to internalize less.

*  _case 2 will only happen if there is exactly one definition of the value (i.e. in exactly one module), as duplicate defs are result in the value being marked exported._  
* _Therefore, only internalize linkonce/weak if there is a single copy, that is prevailing in this IR module._ 

https://github.com/llvm/llvm-project/pull/164861