Treat conditionally executed non-pure calls as errors

  This replaces the support for user defined error functions by a
  heuristic that tries to determine if a call to a non-pure function
  should be considered "an error". If so the block is assumed not to be
  executed at runtime. While treating all non-pure function calls as
  errors will allow a lot more regions to be analyzed, it will also
  cause us to dismiss a lot again due to an infeasible runtime context.
  This patch tries to limit that effect. A non-pure function call is
  considered an error if it is executed only in conditionally with
  regards to a cheap but simple heuristic.

llvm-svn: 249611