This prevents the emission of DAG-generated vreg definitions after a
tail call be dropping them entirely (on the grounds that nothing could
use them anyway, and they interfere with O0 CodeGen).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185754 91177308-0d34-0410-b5e6-96231b3b80d8
