Merging r367941:

------------------------------------------------------------------------
r367941 | reames | 2019-08-06 00:34:59 +0200 (Tue, 06 Aug 2019) | 5 lines

Add a note to the release not about a potentially breaking optimization

This has come up twice already (once in pr42763 and once in the commit thread), so give warning of a new way in which UB can result in unexpected program behavior.


------------------------------------------------------------------------

llvm-svn: 367997