[ScopBuilder] Move verifyInvariantLoads function from ScopInfo. NFC.

Refactor Scop and ScopBuilder class. Move verifyInvariantLoads from Scop
class to ScopBuilder class.

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

Differential Revision: https://reviews.llvm.org/D62628

llvm-svn: 362258