Record assumptions first and add them later

There are three reasons why we want to record assumptions first before we
add them to the assumed/invalid context:

  1) If the SCoP is not profitable or otherwise invalid without the
     assumed/invalid context we do not have to compute it.
  2) Information about the context are gathered rather late in the SCoP
     construction (basically after we know all parameters), thus the user
     might see overly complicated assumptions to be taken while they would
     have been simplified later on.
  3) Currently we cannot take assumptions at any point but have to wait,
     e.g., for the domain generation to finish. This makes wrapping
     assumptions much more complicated as they need to be and it will
     have a similar effect on "signed-unsigned" assumptions later.

llvm-svn: 266068