[llvm] [SandboxVec] Move seed collection into its own separate pass (PR #127132)

[via llvm-commits]

vporpo wrote:

There was a use-after-free caused by calling `R.clearAux()` in `SeedCollection.cpp`, because it goes through the instructions in region's Aux dropping their metadata, but these may have been erased. So I added `Region::removeFromAux()` which gets called whenever an instruction gets deleted.

https://github.com/llvm/llvm-project/pull/127132