[llvm] DAG: Preserve poison in some cast folds (PR #225403)

[Matt Arsenault via llvm-commits]

arsenm wrote:

The constant folding combiners unfortunately aren't redundant with the getNode folds, though the difference is hard to observe. e.g., if something does a ReplaceAllUsesWith with a constant value, these unfolded nodes will appear. Though I'd probably prefer to remove all of the magic folds in getNode, which does way too much 

https://github.com/llvm/llvm-project/pull/225403