IR: Make stripDebugInfo robust against (invalid) empty basic blocks

Since stripDebugInfo runs before the verifier when reading IR, we can
end up in a situation where we read some invalid IR but don't know its
invalid yet. Before this patch we would crash in stripDebugInfo when
given IR with a completely empty basic block, and after we get a nice
error from the verifier instead.

llvm-svn: 311202