[llvm] [Reland][ValueTracking] Improve Bitcast handling to match SDAG (PR #145223)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Yes, an InstSimplify test is preferable thanks - make sure you give the test a better name and add a comment above it describing its purpose. Hopefully it can be added to test\Transforms\InstSimplify\shift-knownbits.ll

https://github.com/llvm/llvm-project/pull/145223