[Release Notes] Add release note for "-fmerge-all-constants"

Summary:
Add note that "-fmerge-all-constants" is not applied as default
anymore.

Reviewers: rjmccall, rsmith, chandlerc

Subscribers: llvm-commits, thakis

Differential Revision: https://reviews.llvm.org/D45388

llvm-svn: 329457

llvm/docs/ReleaseNotes.rst

@@ -58,6 +58,9 @@ Non-comprehensive list of changes in this release
   information available in LLVM to statically predict the performance of
   machine code for a specific CPU.
 
+* The optimization flag to merge constants (-fmerge-all-constants) is no longer
+  applied by default.
+
 * Note..
 
 .. NOTE