[ScopBuilder] Move hoistInvariantLoads to ScopBuilder. NFC.

Refactor Scop and ScopBuilder class:

1. Move hoistInvariantLoads function from Scop to ScopBuilder class.

2. Private functions (addInvariantLoads, getNonHoistableCtx) are moved
   to public section of Scop class. hoistInvariantLoads function
   references these functions. These functions will be moved to
   ScopBuilder as well in the next steps.

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

Differential Revision: https://reviews.llvm.org/D62925

llvm-svn: 363121