[llvm] [DAG] Add ComputeNumSignBits(FREEZE(X)) handling (PR #161507)

[Björn Pettersson via llvm-commits]

bjope wrote:

I think it is a bit unclear why this is needed. If the operand is known not to be undef/poison, then I would assume that the freeze is removed. So is it some kind of internal phase order at isel, or are we keeping freeze for some reason?

https://github.com/llvm/llvm-project/pull/161507