Dissallow non-empty metadata for invariant.group

Summary:
This feature is not needed, but it might be usefull in the future
to use metadata to mark what which function should support it
(and strip it when not).

Reviewers: rsmith, sanjoy, amharc, kuhar

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45419

llvm-svn: 332787
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index b8da194..614c63a 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -109,6 +109,8 @@
 
 * invariant.group.barrier has been renamed to launder.invariant.group.
 
+* invariant.group metadata can now refer only empty metadata nodes.
+
 Changes to the ARM Backend
 --------------------------