[llvm] [SandboxIR][Region] Auxiliary vector metadata now requires a region (PR #137443)

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

Hm, it's unfortunate that we have to break the invariant that the cost of the region is the cost of the instructions in it. So now we can have an instruction added to a Region just because it's in an aux vector, but it doesn't count towards the cost. What if later we decide to expand the Region by adding that instruction "for real" and want its cost to be counted?

Is there another way to encode the association between a Region and its aux vector in metadata so we don't have to do this?

https://github.com/llvm/llvm-project/pull/137443