Fix a latent bug where, after emitting an expression statement, we would

Fix a latent bug where, after emitting an expression statement, we would
delete the block we began emitting into if it had no predecessors.  We never
want to do this, because there are several valid cases during statement
emission where an existing block has no known predecessors but will acquire
some later.  The case in my test case doesn't inherently fall into this
category, because we could safely emit the case-range code before the statement
body, but there are examples with labels that can't be fallen into
that would also demonstrate this bug.

rdar://problem/8837067

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123303 91177308-0d34-0410-b5e6-96231b3b80d8