Check overflows in RTCs and bail accordingly

  We utilize assumptions on the input to model IR in polyhedral world.
  To verify these assumptions we version the code and guard it with a
  runtime-check (RTC). However, since the RTCs are themselves generated
  from the polyhedral representation we generate them under the same
  assumptions that they should verify. In other words, the guarantees
  that we try to provide with the RTCs do not hold for the RTCs
  themselves. To this end it is necessary to employ a different check
  for the RTCs that will verify the assumptions did hold for them too.

Differential Revision: http://reviews.llvm.org/D20165

llvm-svn: 269299