Bring back "Assert that we have all use/users in the getters."

This reverts commit r257751, bringing back r256105.

The problem the assert found was fixed in r257915.

Original commit message:

Assert that we have all use/users in the getters.

An error that is pretty easy to make is to use the lazy bitcode reader
and then do something like

if (V.use_empty())

The problem is that uses in unmaterialized functions are not accounted
for.

This patch adds asserts that all uses are known.

llvm-svn: 257920