[NFC][ScopBuilder] Move addUserAssumptions to ScopBuilder

Scope of changes:
1) Moved addUserAssumptions function to ScopBuilder class.
2) Moved buildConditionSets functions to polly namespace.
3) Moved getRepresentingInvariantLoadSCEV to public section of the Scop class

Differential Revision: https://reviews.llvm.org/D65241

llvm-svn: 368089