[globalisel] Rename GISelChangeObserver's erasedInstr() to erasingInstr() and related nits. NFC

Summary:
There's little of interest that can be done to an already-erased instruction.
You can't inspect it, write it to a debug log, etc. It ought to be notification
that we're about to erase it. Rename the function to clarify the timing of the
event and reflect current usage.

Also fixed one case where we were trying to print an erased instruction.

Reviewers: aditya_nandakumar

Reviewed By: aditya_nandakumar

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D55611

llvm-svn: 348976