Split part of isKilled() into a separate function for use elsewhere.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175726 91177308-0d34-0410-b5e6-96231b3b80d8
Make another kill check LiveIntervals-aware.

This brings the number of remaining failures in 'make check' without
LiveVariables down to 39, with 1 unexpectedly passing test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175727 91177308-0d34-0410-b5e6-96231b3b80d8