We have an assertion that checks that we do not encode null values into the

We have an assertion that checks that we do not encode null values into the
.bc file if they are supposed to be implicit.  This is cool, except that it
checked *after* constant expr folding: improving constant expr folding could
cause the .bc reader to assert out on old .bc files.  Move the check so that
it checks all simple constants, but no constantexprs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27480 91177308-0d34-0410-b5e6-96231b3b80d8