[llvm] [ValueLattice] Reset the count of range extensions when merging an undef with a constant range. (PR #77307)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

It's not really clear to me that this change is right. How NumRangeExtensions should behave when merging lattice values isn't clear to me. The proposed behavior is asymmetric -- but maybe that's okay?

https://github.com/llvm/llvm-project/pull/77307