[A57FPLoadBalancing] Ignore <def>s when checking if a chain may be killed.

Defs are seen before uses, so a def without the kill flag doesn't necessarily
mean that the register is not killed on that instruction. It may be killed
in a later use operand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217689 91177308-0d34-0410-b5e6-96231b3b80d8