InvariantEquivClassTy: Use struct instead of 4-tuple to increase readability

Summary:
With a struct we can use named accessors instead of generic std::get<3>()
calls. This increases readability of the source code.

Reviewers: jdoerfert

Subscribers: pollydev, llvm-commits

Differential Revision: http://reviews.llvm.org/D21955

llvm-svn: 275056