JIT: Consolidate block pred iterator logic

[amanasifkhalid]

Move most of the nested iterator class logic in PredEdgeList/PredBlockList to a generic base class, and implement subclasses that specify the yield type (flow edge or basic block) and whether edits are allowed. This allows us to add a pred edge iterator that tolerates edits with minimal code duplication.

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[amanasifkhalid]

From the TP diffs, I suspect MSVC isn't devirtualizing the operator* calls, though it probably makes sense to just move this method to the subclasses entirely, and remove the IteratorType template variable.

[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. No diffs. Thanks!