Tag build swift-DEVELOPMENT-SNAPSHOT-2017-02-10-a
Merge remote-tracking branch 'origin/swift-4.0-branch' into stable

* origin/swift-4.0-branch:
  ReleaseNotes.rst: Add a section about .mir testing
  RegisterCoalescer: Fix joinReservedPhysReg()
  RegisterCoalescer: Cleanup joinReservedPhysReg(); NFC