MachineCopyPropagation: Do not consider undef operands as clobbers
This was originally introduced in r278321 to work around correctness
problems in the ExecutionDepsFix pass; Probably also to keep the
performance benefits of breaking the false dependencies which of course
also affect undef operands.
ExecutionDepsFix has been improved here recently (see for example
r278321) so we should not need this exception any longer.
Differential Revision: https://reviews.llvm.org/D29525