[ARM] Fix bug in ARMLoadStoreOptimizer when kill flags are missing.

Summary:
ARMLoadStoreOpt::FixInvalidRegPairOp() was only checking if one of the
load destination registers to be split overlapped with the base register
if the base register was marked as killed.  Since kill flags may not
always be present, this can lead to incorrect code.

This bug was exposed by my MachineCopyPropagation change D30751 breaking
the sanitizer-x86_64-linux-android buildbot.

Also clean up some dead code and add an assert that a register offset is
never encountered by this code, since it does not handle them correctly.

Reviewers: MatzeB, qcolombet, t.p.northover

Subscribers: aemerson, javed.absar, kristof.beyls, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D37164

llvm-svn: 311907