[globalisel] Fix iterator invalidation in the extload combines

Summary:
Change the way we deal with iterator invalidation in the extload combines as it
was still possible to neglect to visit a use. Even worse, it happened in the
in-tree test cases and the checks weren't good enough to detect it.

We now take a cheap copy of the use list before iterating over it. This
prevents iterator invalidation from occurring and has the nice side effect
of making the existing schedule-for-erase/schedule-for-insert mechanism
moot.

Reviewers: aditya_nandakumar

Reviewed By: aditya_nandakumar

Subscribers: rovka, kristof.beyls, javed.absar, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61813

llvm-svn: 363616