update_mir_test_checks: Handle empty liveins

An empty livein block doesn't make much sense (why not just omit it?)
but they're legal and some tests have them, so its best to handle it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316089 91177308-0d34-0410-b5e6-96231b3b80d8