RegisterCoalescer: rewrite eliminateUndefCopy().

This also fixes problems with undef copies of subregisters. I can't
attach a testcase for that as none of the targets in trunk has
subregister liveness tracking enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224560 91177308-0d34-0410-b5e6-96231b3b80d8