Fix for r180193 - MI Sched: eliminate local vreg.

Fixes PR15838. Need to check for blocks with nothing but dbg.value.

I'm not sure how to force this situation with a unit test. I tried to
reduce the test case in PR15838 (1k lines of metadata) but gave up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180227 91177308-0d34-0410-b5e6-96231b3b80d8