[CGP] Drop nneg flag when moving zext past instruction (#72103)

Fix the issue by not reusing the zext at all. The code already handles
creation of new zexts if more than one is needed. Always use that
code-path instead of trying to reuse the old zext in some case.
(Alternatively we could also drop poison-generating flags on the old
zext, but it seems cleaner to not reuse it at all, especially if it's
not always possible anyway.)

Fixes https://github.com/llvm/llvm-project/issues/72046.