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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Can you do something with this: https://alive2.llvm.org/ce/z/yaFWKg - 

instcombine will try to replace a ashr with lshr if it can confirm all the signbits are known zero

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