[perl #58410] [TODO] Deprecate n_* variants of the math opcodes

[Allison Randal (via RT) <parrotbug-followup@parrotcode.org>]

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Briefly discussed on the phone with Patrick, Jerry, and chromatic: The 
versions of the math opcodes that modify an existing destination PMC 
instead of creating a new destination PMC are not useful to HLLs, 
because they make assumptions about assignment semantics that don't hold 
true for all (or possibly even any) HLLs. Code generated from PCT takes 
the result of the math op as a temporary result value, and then performs 
a separate assignment operation to the HLL result variable, following 
the HLLs semantics for assignment.

The plan is to make the regular variants (like 'add') create a new 
destination PMC, and then deprecate the old n_* variants (like 'n_add').

Allison