Handle a COPY with undef source operand in LowerCopy()

Llvm-stress discovered that a COPY may end up in ExpandPostRA::LowerCopy()
with an undef source operand. It is not possible for the target to handle
this, as this flag is not passed to TII->copyPhysReg().

This patch solves this by treating such a COPY as an identity COPY.

Review: Matthias Braun
https://reviews.llvm.org/D32892

llvm-svn: 302877