Remove irreducible control flow from test case

The test case we look at does not necessarily require irreducible control flow,
but a normal loop is sufficient to create a non-affine region containing more
than one basic block that dominates the exit node. We replace this irreducible
control flow with a normal loop for the following reasons:

  1) This is easier to understand
  2) We will subsequently commit a patch that ensures Polly does not process
     irreducible control flow.

Within non-affine regions, we could possibly handle irreducible control flow.

llvm-svn: 258496