[ScopInfo] Avoid infinite loop during schedule construction

Our current scop modeling enters an infinite loop when trying to model code
that has unreachable instructions (e.g.,
test/ScopInfo/BoundChecks/single-loop.ll), as the number of basic blocks
returned by the LLVM Loop* does not include unreachable basic blocks that
branch off from the core loop body. This arises for example in the following
piece of code:

  for (i = 0; i < N; i++) {
    if (i > 1024)
      abort();            <- this abort might be translated to an
                             unreachable

    A[i] = ...
  }

This patch adds these unreachable basic blocks in our per loop basic block
count to ensure that the schedule construction does not assume a loop has been
processed completely, despite certain unreachable basic blocks still remaining.

The infinite loop is only observable in combination with
https://reviews.llvm.org/D12676 or a similar patch.

llvm-svn: 297156