R600/SI: Check for empty stack in SIAnnotateControlFlow::isTopOfStack

Fixes assertion failure in newly added lit test. Might just be a bandaid that
needs to be revisited.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175139 91177308-0d34-0410-b5e6-96231b3b80d8