Jit: Conditional Escape Analysis and Cloning

[AndyAyersMS]

Enhance escape analysis to determine if an object escapes only under failed GDV tests. If so, clone to create a path of code so that the object doesn't escape, and then stack allocate the object.

More details in the included document.

Contributes to #108913

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[AndyAyersMS]

@dotnet/jit-contrib PTAL

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[amanasifkhalid]

Nice writeup! I haven't looked at the code yet, though you should probably get someone else to look at it

[AndyAyersMS]

One of the libraries failures is that we're not updating the enclosing EH extents properly.

Assertion failed 'found && "BBJ_EHFINALLYRET predecessor of block that doesn't follow a BBJ_CALLFINALLY!"' in 'System.IO.File:InternalWriteAllLines(System.IO.StreamWriter,System.Collections.Generic.IEnumerable`1[System.String])' during 'Allocate Objects' (IL size 84; hash 0xfef048f0; Tier1)

fgCloneTryRegion assumes the cloned try will be lexically after the original, so any EH extent that ended at the original now ends at the clone. But here we're often putting the clone before the original, and so we get the enclosing extent wrong.

It should be possible to reorder these and put the clone after, but the EH lexicality constraints have been a consistent source of trouble.