[MachineOutliner] Recommit r312194, missed optimization remarks

Before, this commit caused a buildbot failure:

http://bb.pgr.jp/builders/test-llvm-i686-linux-RA/builds/6026/steps/test_llvm/logs/LLVM%20%3A%3A%20CodeGen__AArch64__machine-outliner-remarks.ll

This was caused by the Key value in DiagnosticInfoOptimizationBase being
deallocated before emitting the remarks defined in MachineOutliner.cpp. As of
r312277 this should no longer be an issue.
 

llvm-svn: 312280