[MachineCopyPropagation] Rework how we manage RegMask clobbers

Instead of updating the CopyTracker's maps each time we come across a
RegMask, defer checking for this kind of interference until we're
actually trying to propagate a copy. This avoids the need to
repeatedly iterate over maps in the cases where we don't end up doing
any work.

This is a slight compile time improvement for MachineCopyPropagation
as is, but it also enables a much bigger improvement that I'll follow
up with soon.

Differential Revision: https://reviews.llvm.org/D52370

llvm-svn: 342940