Avoid generation of dead code in a few more situations.

Avoid generation of dead code in a few more situations.
 - Emit variable declarations as "simple", we want to avoid forcing the creation
   of a dummy basic block, but still need to make the variable available for
   later use.

 - With that, we can now skip IRgen for other unreachable statements (which
   don't define a label).

 - Anders, I added two fixmes on calls to EmitVLASize, can you check them?

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@76361 91177308-0d34-0410-b5e6-96231b3b80d8