Only add user assumptions on known parameters [NFC]

  Before, assumptions derived from llvm.assume could reference new
  parameters that were not known to the SCoP before. These were neither
  beneficial to the representation nor to the user that reads the
  emitted remark. Now we project them out and keep only user assumptions
  on known parameters. Nevertheless, the new parameters are still part
  of the SCoPs parameter space as the SCEVAffinator currently adds them
  on demand.

llvm-svn: 267441