Track invalid domains not invalid contexts for statements

  The invalid context is not enough to describe the parameter constraints under
  which a statement is not modeled precisely. The reason is that during the
  domain construction the bounds on the induction variables are not known but
  needed to check if e.g., an overflow can actually happen. To this end we
  replace the invalid context of a statement with an invalid domain. It is
  initialized during domain construction and intersected with the domain once
  it was completely build. Later this invalid domain allows to eliminate
  falsely assumed wrapping cases and other falsely assumed mismatches in the
  modeling.

llvm-svn: 267286