[ScopHelper] Provide support for recognising collective invariant loads

Summary: This patch aims to provide support for detecting load patterns which are collectively invariant but right now `isHoistableLoad()` is checking each load instruction individually which cannot detect the load pattern as a whole.

Patch by: Sahil Girish Yerawar

Reviewers: bollu, philip.pfaffe, Meinersbur

Reviewed By: philip.pfaffe, Meinersbur

Differential Revision: https://reviews.llvm.org/D48026

llvm-svn: 335949