[ScopBuilder] Move buildInvariantEquivalenceClasses function from ScopInfo. NFC.

Refactor Scop and ScopBuilder class. Move
buildInvariantEquivalenceClasses function from Scop class to ScopBuilder
class.

Patch by: Dominik Adamski <adamski.dominik@gmail.com>

Differential Revision: https://reviews.llvm.org/D62351

llvm-svn: 361902