JIT: Add pred block iterator that tolerates pred list modifications

[amanasifkhalid]

Follow-up to #99362. fgRedirectTargetEdge modifies the predecessor lists of the old and new successor blocks, so if we want to be able to use it in fgReplaceJumpTarget, we need a pred block iterator that is resilient to these changes, as we frequently call the latter method while iterating predecessors.

I haven't found the need to update the pred edge iterator to allow updates too, but I imagine it would look the same as the pred block iterator.

fgTailMergeThrows looks a bit awkward right now. After this change is merged, I plan to add a new method similar to fgRedirectTargetEdge for BBJ_COND blocks, and leverage it in fgReplaceJumpTarget. Since that new method will preserve edge weights, we won't lose anything in fgTailMergeThrows by just calling fgReplaceJumpTarget. I think that will be a good opportunity to clean up fgTailMergeThrows by getting rid of all its helpers for updating edges.

cc @dotnet/jit-contrib, @AndyAyersMS PTAL.

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[AndyAyersMS]

I like this direction, but would prefer that the nature of the iterator be obvious from its name..

Maybe something like:

class PredBlockList
{
public:

   static PredBlockList PredBlocks() const
   {
      return PredBlockList(bbPreds, /*allowEdits*/ false);
   }

   static PredBlockList PredBlocksEditing() const
   {
      return PredBlockList(bbPreds, /*allowEdits*/ true);
   }

private:

   PredBlockList(FlowEdge* list, bool allowEdits) {...};
};

[AndyAyersMS]

This seems to be modifying live state under debug, which I think we should avoid.

[amanasifkhalid]

@AndyAyersMS I separated out the allowEdits=true path in operator++. It has a bit of duplicated code now, but I think it makes it clearer that m_next is always updated for editing iterators, but only updated for non-editing iterators in Debug, so there shouldn't be any newly-introduced cases where we update state in Debug only. I also added PredBlocksEditing, but kept the templated pattern with the hope that we can avoid some branches in the iterator logic -- are you ok with this approach?

[AndyAyersMS]

This looks great.

[amanasifkhalid]

Thanks for the reviews! I should note that this change has small diffs from slight changes in edge weights.