[GVNHoist] don't hoist callbr users into the callbr's block

This isn't safe to do.

Link: https://github.com/llvm/llvm-project/issues/53562
Fixes: https://github.com/llvm/llvm-project/issues/61023

Reviewed By: efriedma, nikic

Differential Revision: https://reviews.llvm.org/D144927