it seems i may have been mistaken in my assumption that there is a way to do this based on the fact that there are increasing 'identifier' values printed in the SILGen output. i was informed that this is a byproduct of the logic in SILPrinter which also does a traversal of all blocks/instructions to generate those values.
so, i will assume this isn't possible to do without some form of local cache derived from walking the instructions at least once.