[llvm] [JumpThreading] Refractor redirectValuesFromPredecessorsToPhi: avoid unnecessary map resizing in gatherIncomingValuesToPhi (PR #173596)

[via llvm-commits]

int-zjt wrote:

Seems clang-format will check the UndefValue usage in code diff, but it is hard to avoid using them here.

https://github.com/llvm/llvm-project/pull/173596