[FunctionAttrs] Fix incorrect nonnull inference for non-inbounds GEP (#91180)

For inbounds GEPs, if the source pointer is non-null, the result must
also be non-null. However, this does not hold for non-inbounds GEPs.

Fixes https://github.com/llvm/llvm-project/issues/91177.

(cherry picked from commit f34d30cd)