[AssumeBundles] adapt Assumption cache to assume bundles

Summary: change assumption cache to store an assume along with an index to the operand bundle containing the knowledge.

Reviewers: jdoerfert, hfinkel

Reviewed By: jdoerfert

Subscribers: hiraditya, mgrang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77402