[AssumeBundles] Add API to fill a map from operand bundles of an llvm.assume.

Summary: This patch adds a new way to query operand bundles of an llvm.assume that is much better suited to some users like the Attributor that need to do many queries on the operand bundles of llvm.assume. Some modifications of the IR like replaceAllUsesWith can cause information in the map to be outdated, so this API is more suited to analysis passes and passes that don't make modification that could invalidate the map.

Reviewers: jdoerfert, sstefan1, uenoku

Reviewed By: jdoerfert

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75020