Statically analyze likely and unlikely blocks based on metadata

The builtin_expect(), and C++20's likely, unlikely attributes assign branch_weights to annotated branches.

This patch adds the the ability to query branch !prof metadata and improve static analysis based on that.

Fixes: https://github.com/llvm/llvm-project/issues/64998

Reviewers: tejohnson, efriedma

Differential Revision: https://reviews.llvm.org/D159336