[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329457 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed