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

[via llvm-commits]

goldsteinn wrote:

This is notably test free and not an NFC. I didn't see any test changes from these and *hope* it won't be necessary to add the 100 odd tests for all the new cases.

My thinking is these cases seem to have just been oversights during original implementation, and if the original commit had had `DemandedElt` propagation, it probably would have gotten on with normal tests.

But LMK.

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