[AArch64][GlobalISel] Fix llvm.returnaddress(0) selection when LR is clobbered.

[AArch64][GlobalISel] Fix llvm.returnaddress(0) selection when LR is clobbered.

The code was originally ported from SelectionDAG, which does CSE behind the scenes
automatically. When copying the return address from LR live into the function, we
need to make sure to use the single copy on function entry. Any later copy from LR
could be using clobbered junk.

Implement this by caching the copy in the per-MF state in the selector.

Should hopefully fix the AArch64 sanitiser buildbot failure.