[llvm] [InitUndef] Don't use largest super class (PR #107885)

[Piyou Chen via llvm-commits]

https://github.com/BeMg approved this pull request.

The initial idea to identify the largest class is to map the various register classes to four init pseudos.

LGTM on removing it.

https://github.com/llvm/llvm-project/pull/107885