[llvm] [InstCombine] SimplifyDemandedBits: add basic bitcast handling, Fixes #173780 (PR #173865)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Try tests with a single bitcast from v1i32 to i32 instead, double bitcasts will fold away 

https://github.com/llvm/llvm-project/pull/173865