Revert r314879 "[UnreachableBlockElim] Use COPY if PHI input is undef"

Build-bots broke on the new testcase. I'll investigate and fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314880 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed