[globalisel] Add GISelChangeObserver::changingInstr()

Summary:
In addition to knowing that an instruction is changed. It's also useful to
know when it's about to change. For example, it might print the instruction so
you can track the changes in a debug log, it might remove it from some queue
while it's being worked on, or it might want to change several instructions as
a single transaction and act on all the changes at once.

Added changingInstr() to all existing uses of changedInstr()

Reviewers: aditya_nandakumar

Reviewed By: aditya_nandakumar

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D55623

llvm-svn: 348992