SIL: fix a complexity problem in the ownership verifier
Computing dominance relation between instructions in the same block was done with linear search, e.g. when checking if a value-use is before its lifetime ending instruction.
This resulted in quadratic complexity and very long compile times for very large basic blocks.
Now we do the dominance check with pre-computed instruction indices, which is O(0) instead of O(n).
https://github.com/swiftlang/swift/issues/86663
rdar://168511262
