[llvm] [ValueTracking] Consistently propagate `DemandedElts` in ValueTracking functions (PR #99080)

[via llvm-commits]

goldsteinn wrote:

> Could you please check whether this has any compile-time impact? (I think the impact might be positive because we don't have to construct a dummy DemandedElts all the time?)
> 
Seems to be a minor improvement: https://llvm-compile-time-tracker.com/compare.php?from=769952d72ff383e0e5fda46802fb8717712784d3&to=0df283c8117daebd019927aff76f78f43cfb5569&stat=instructions%3Au

https://github.com/llvm/llvm-project/pull/99080