JIT: Codegen should unset register assignments after defining nodes that will be used from spill temps

[jakobbotsch]

It is confusing that nodes that end up being used directly from their spill location still end up with a register assignment set when we get to their use. I do not think that register assignment has anything meaningful, so as part of the spill codegen should be able to unset it.

Context: #105833 (comment)

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[jakobbotsch]

I do not think that register assignment has anything meaningful, so as part of the spill codegen should be able to unset it.

That turns out not to be entirely true -- we use it as an index into the list of spill temps to find the right one when we get to the use. So it's not just clearing out the register on the use, we would also need to switch that logic to some form of map instead.