[BFI] Use CallbackVH to notify BFI about deletion of basic blocks

With AssertingVHs instead of bare pointers in
BlockFrequencyInfoImpl::Nodes (but without CallbackVHs) ~1/36 of all
tests ran by make check fail. It means that there are users of BFI that
delete basic blocks while keeping BFI. Some of those transformations add
new basic blocks, so if a new basic block happens to be allocated at
address where an already deleted block was and we don't explicitly set
block frequency for that new block, BFI will report some non-default
frequency for the block even though frequency for the block was never
set. Inliner is an example of a transformation that adds and removes BBs
while querying and updating BFI.
With this patch, thanks to updates via CallbackVH, BFI won't keep stale
pointers in its Nodes map.

Reviewers: davidxl, yamauchi, asbirlea, fhahn, fedor.sergeev

Reviewed-By: asbirlea, davidxl

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75341