[ARM] Remove dead instructions before creating VPT block bundles

We remove VPNOT instructions in VPT blocks as we create them, turning
them into else predicates. We don't remove the dead instructions until
after the block has been created though. Because the VPNOT will have
killed the vpr register it used, this makes finalizeBundle add internal
flags to the vpr uses of any instructions after the VPNOT. These
incorrect flags can then confuse what is alive and what is not, leading
to machine verifier problems.

This patch removes them earlier instead, before the bundle is finalized
so that kill flags remain valid.

Differential Revision: https://reviews.llvm.org/D92227