Translate SCEVs to isl_pw_aff and their invalid domain

  The SCEVAffinator will now produce not only the isl representaiton of
  a SCEV but also the domain under which it is invalid. This is used to
  record possible overflows that can happen in the statement domains in
  the statements invalid domain. The result is that invalid loads have
  an accurate execution contexts with regards to the validity of their
  statements domain. While the SCEVAffinator currently is only taking
  "no-wrapping" assumptions, we can add more withouth worrying about the
  execution context of loads that are optimistically hoisted.

llvm-svn: 267288