Fixed bug where the CFG would fail to build when an 'if' statement had

an empty then or else block (or contained only ';' statements).

For example, we now handle the following:

int empty_else() { if (0) { int a; } else ; }
int empty_then() { if (0) ; else { int a; } }

Thanks to Nico Weber for spotting this problem.

llvm-svn: 41617