Change error to warning when a profile cannot be found.

When the profile for a function cannot be applied, we use to emit an
error. This seems extreme. The compiler can continue, it's just that the
optimization opportunities won't include profile information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220386 91177308-0d34-0410-b5e6-96231b3b80d8