Add some release notes about the fix for PR26774

As suggested by Chandler on the review thread for D18634.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267239 91177308-0d34-0410-b5e6-96231b3b80d8

Author: sanjoy

docs/ReleaseNotes.rst

@@ -87,6 +87,16 @@ Changes to the LLVM IR
   ``llvm.masked.gather`` and ``llvm.masked.scatter`` were introduced to the
   LLVM IR to allow selective memory access for vector data types.
 
+Changes to LLVM's IPO model
+---------------------------
+
+LLVM no longer does inter-procedural analysis and optimization (except
+inlining) on functions with comdat linkage.  Doing IPO over such
+functions is unsound because the implementation the linker chooses at
+link-time may be differently optimized than the one what was visible
+during optimization, and may have arbitrarily different observable
+behavior.  See `PR26774 <http://llvm.org/PR26774>`_ for more details.
+
 Changes to the ARM Backend
 --------------------------