JIT: Refine post-dominance check in strength reduction

[jakobbotsch]

When strength reduction has to find an insertion point for the new primary IV update it needs to find a block that post-dominates all the users of the IV. This was using optReachable before, but that is conservative since it finds paths that may exit the loop. This implements a more precise check.

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[jakobbotsch]

/azp run runtime-coreclr jitstress, runtime-coreclr libraries-jitstress

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[jakobbotsch]

/azp run runtime-coreclr libraries-jitstress, runtime, dotnet-linker-tests

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Diffs with strength reduction enabled show a lot of new cases where strength reduction is now considered legal.

[AndyAyersMS]

Looks good.

I wonder if would be more efficient to walk backwards from the loop head backedges, seeing if you can reach block w/o first reaching postDominator?

[jakobbotsch]

I wonder if would be more efficient to walk backwards from the loop head backedges, seeing if you can reach block w/o first reaching postDominator?

Hmm, that could very well be. We could also check all of the users simultaneously instead of just checking them one at a time. I'll keep this in mind for when I start looking at TP.