[ValueTracking] Ignore poison values in `computeKnownBits` (#72683)

This patch handles `poison` elements of non-splat vectors in
`computeKnownBits`. It addresses test changes after I delete the
duplicate logic in https://github.com/llvm/llvm-project/pull/72535.

See also @nikic's comment:
https://github.com/llvm/llvm-project/pull/72535#pullrequestreview-1736991557