LoopSimplify: add flag to prohibit separation of loops

* Loops with multiple backedges are potentially separated
  by creating a new nested loop. This may enable further optimizations
  for the separated loops, but complicates manual annotaions of loop
  bounds, e.g. for analysis with an external tool.
* It also hampers generation of single-path code.
* Once header loop bounds are translated into markers and corresponding
  constraints, this transformation may not be harmful anymore.
Disallow duplication of BBs containing a loopbound intrinsic.

* Handled in the CodeMetrics interface.
PMLExport: prettier printing of warnings when exporting loopbounds