[llvm] [DAGCombiner] Ensure poison-generating flags are stripped in `freeze` op (PR #114582)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> When combining freeze in DAG, poison-generating flags were previously not fully removed as intended. This change explicitly drops these flags from freeze operand, ensuring a fixed, non-poisonous value at this point in computation.

Sorry if I've missed something (long day), but where was it intended that the poison flags should be removed? 

https://github.com/llvm/llvm-project/pull/114582