While this doesn't appear to help with the perf issue being exposed by D84108,
the function as-is is very weird, convoluted, and what's worse, recursive.
There was no need for SpeculativelyAvaliableAndUsedForSpeculation,
tri-state choice is enough. We don't even ever check for that state.
The basic idea here is that we need to perform a depth-first traversal of the
predecessors of the basic block in question, either finding a preexisting
state for the block in a map, or inserting a "placeholder" SpeculativelyAvaliable,
If we encounter an Unavaliable block, then we need to give up search,
and back-propagate the Unavaliable state to the each successor of said block,
more specifically to the each SpeculativelyAvaliable we've just created.
However, if we have traversed entirety of the predecessors and have not
encountered an Unavaliable block, then it must mean the value is fully available.
We could update each inserted SpeculativelyAvaliable into a Avaliable,
but we don't need to, as assertion excersizes, because we can assume that if
we see an SpeculativelyAvaliable entry, it is actually Avaliable,
because during the time we've produced it, if we would have found that it
has an Unavaliable predecessor, we would have updated it's successors,
including this block, into Unavaliable