I thought `reverse(silFn)` would do a post-order walk, but I was wrong.
This patch cuts the number of iterations to propagate coldness from
3-4 down to 2 in a few of the simple regression test cases. At least on
macOS (as the stdlib can vary per platform).
The old analysis pass doesn't take into account profile data, nor does
it consider post-dominance. It primarily dealt with _fastPath/_slowPath.
A block that is dominated by a cold block is itself cold. That's true
whether it's forwards or backwards dominance.
We can also consider a call to any `Never` returning function as a
cold-exit, though the block(s) leading up to that call may be executed
frequently because of concurrency. For now, I'm ignoring the concurrency
case and assuming it's cold. To make use of this "no return" prediction,
use the `-enable-noreturn-prediction` flag, which is currently off by
default.
