[Verifier] Check that !nonnull metadata is empty

!nonnull expectes an empty metadata argument, so check that this
is the case in the verifier. This came up as a problem in
https://reviews.llvm.org/D141386.

This requires dropping the verifier call in the compatibility-6.0.ll
test (which is not present in any of the other bitcode compatibility
tests). The original input unfortunately used typo'd nonnull
metadata.