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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Please can you rebase - I've added the test to trunk so this patch can show the diff more easily.

The next step is add to add more test coverage - if you comment out the DAG equivalent code and see the test changes you might be able to adapt some of those tests to create new IR tests?

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