[llvm] [ValueTracking] Improve `Bitcast` handling to match SDAG (PR #125935)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Sure - I'd probably recommend renaming `pr125228.ll` to `bitcast-known-bits.ll` and instead mention PR125228 in a comment at near the top of the file (after the RUN lines etc.)

https://github.com/llvm/llvm-project/pull/125935