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

[Simon Pilgrim via llvm-commits]

RKSimon 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 :( 

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