cmd/compile: simplify PGO hot caller/callee computation
Currently, we use CDF to compute a weight threshold and then use
the weight threshold to determine whether a call site is hot. As
when we compute the CDF we already have a list of hot call sites
that make up the given percentage of the CDF, just use that list.
Also, when computing the CDF threshold, include the very last node
that makes it to go over the threshold. (I.e. if the CDF threshold
is 50% and one hot node takes 60% of weight, we should include that
node instead of excluding it. In practice it rarely matters,
probably only for testing and micro-benchmarks.)