ValueMapper: Don't allow explicit null mappings of Values, NFC

As a follow-up to r123058, assert that there are no null mappings in the
ValueMap instead of just ignoring them when they are there.  There were
a couple of accidental insertions in CloneFunction so I cleaned those up
(caught by testcases).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266565 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed