prevent jump threading from merging blocks when their address is

prevent jump threading from merging blocks when their address is
taken (and used!).  This prevents merging the blocks (invalidating
the block addresses) in a case like this:

#define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })

void foo() {
  printf("%p\n", _THIS_IP_);
  printf("%p\n", _THIS_IP_);
  printf("%p\n", _THIS_IP_);
}

which fixes PR4151.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125829 91177308-0d34-0410-b5e6-96231b3b80d8