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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256105 91177308-0d34-0410-b5e6-96231b3b80d8