Currently we reject to vectorize loops with epilog if trip count less than 16 (as defined by TinyTripCountVectorThreshold) and loop is cold according to profile. Thus in absence of profile summary information we will never vectorize such loops even if cost model can prove profitability. With this change we try to improve the situation by using block execution count information (if available) to judge if loop is hot relatively to function entry.
Please note that I moved related code to getScalarEpilogueLowering. That not only improves maintainability but fixes potential issue of redefining decision made by getScalarEpilogueLowering by short trip count heuristic.
Ideally we should be using loop size and expected perf gain from cost model in addition to loop hotness to make the decision but that would be much bigger change and can be a follow up step.
Note that this change depends on https://reviews.llvm.org/D67690