There is already precedence for doing this via DominanceInfo. The reason I am
doing this is that I need the ownership verifier to be able to be run in Raw SIL
before no return folding has run. This means I need to be able to ignore
unreachable code resulting from SILGen not inserting unreachables after no
return functions.
The reason why SILGen does this is to preserve the source information of the
unreachable code so that we can emit nice errors about unreachable code.
rdar://29791263
