[llvm] [SelectionDAG] Fix bug related to demanded bits/elts for BITCAST (PR #139085)

[Björn Pettersson via llvm-commits]

bjope wrote:

Added some commits showing the attempt of adding a "Do Not Poison Elts" kind of mask to SimplifyDemandedVectorElts (as well as the target node version in a separate step). That feels a bit tricky but reduced the amount of diffs a bit.

https://github.com/llvm/llvm-project/pull/139085