Fix CodeGenPrepare::stripInvariantGroupMetadata

`dropUnknownNonDebugMetadata` takes a list of "known" metadata IDs.  The
only reason it worked at all is that `getMetadataID` returns something
unrelated -- it returns the subclass ID of the receiver (which is used
in `dyn_cast` etc.).  That does not numerically match
`LLVMContext::MD_invariant_group` and ends up dropping `invariant_group`
along with every other metadata that does not numerically match
`LLVMContext::MD_invariant_group`.

llvm-svn: 289973