Andreas Klebinger authored Feb 18, 2019 and Marge Bot committed Oct 16, 2019

For backends maintaining the CFG during codegen
we can now find loops and their nesting level.

This is based on the Cmm CFG and dominator analysis.

As a result we can estimate edge frequencies a lot better
for methods, resulting in far better code layout.

Speedup on nofib: ~1.5%
Increase in compile times: ~1.9%

To make this feasible this commit adds:
* Dominator analysis based on the Lengauer-Tarjan Algorithm.
* An algorithm estimating global edge frequences from branch
probabilities - In CFG.hs

A few static branch prediction heuristics:

* Expect to take the backedge in loops.
* Expect to take the branch NOT exiting a loop.
* Expect integer vs constant comparisons to be false.

We also treat heap/stack checks special for branch prediction
to avoid them being treated as loops.