[AArch64][GlobalISel] Enable copy elision in the pre-legalizer combine and fix a crash.

This enables the simple copy combine that already exists in the CombinerHelper.
However, it exposed a bug in the GISelChangeObserver where it wouldn't clear a
set of MIs to process, and so would end up causing a crash when deleted MIs were
being added to the combiner worklist again.

Differential Revision: https://reviews.llvm.org/D60579

llvm-svn: 358318