Debug Info: clean up usage of Verify.

No functionality change.
It should suffice to check the type of a debug info metadata, instead of
calling Verify. For cases where we know the type of a DI metadata, use
assert.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185249 91177308-0d34-0410-b5e6-96231b3b80d8