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

[Björn Pettersson via llvm-commits]

bjope wrote:

> Its the DemandedElts mask in particular which makes this useful - we can confirm that the specific elements we care about for analysis are not undef/poison, even if other elements are (e.g. due to vector widening, shuffles etc.) which prevents the removal of the freeze entirely.
> 
> Another reason is lack of topological sorting of the DAG, which is going to take years to fix at the current rate of progress :(

Ok. That is what I suspected. Could perhaps be worth mentioning the reasoning for it in the commit msg. But LGTM on the patch.

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