[PM/LoopUnswitch] Fix a bug in the loop block set formation of the new

[PM/LoopUnswitch] Fix a bug in the loop block set formation of the new
loop unswitch.

This code incorrectly added the header to the loop block set early. As
a consequence we would incorrectly conclude that a nested loop body had
already been visited when the header of the outer loop was the preheader
of the nested loop. In retrospect, adding the header eagerly doesn't
really make sense. It seems nicer to let the cycle be formed naturally.
This will catch crazy bugs in the CFG reconstruction where we can't
correctly form the cycle earlier rather than later, and makes the rest
of the logic just fall out.

I've also added various asserts that make these issues *much* easier to
debug.

llvm-svn: 330707