IR: Reserve an MDKind for !llvm.loop; NFC

This reserves an MDKind for !llvm.loop, which allows callers to avoid a
string-based lookup.  I'm not sure why it was missing.

There should be no functionality change here, just a small compile-time
speedup.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264371 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed