[globalisel] Correct string emitted by GISelChangeObserver::erasingInstr()

The API indicates that the MI is about to be erased rather than it has been erased.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353746 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed