Don't unroll loops whose header block's address is taken.

This is part of a futile attempt to not "break" bizzaro
code like this:

 l1:
  printf("l1: %p\n", &&l1);
  ++x;

  if( x < 3 ) goto l1;


Previously we'd fold &&l1 to 1, which is fine per our semantics
but not helpful to the user.

llvm-svn: 125827