Revert the spill_to optimisation
#1747
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In 0511255 I introduced an optimisation that designs to reuse spill slots when possible. That turns out to be broken because, at least in part, input variables can alias on the stack. Here is a chunk from a real trace:
If we reuse
%1s spill slot we might (depending on the trace) corrupt%2.I suspect we can -- if this optimisation is still worth it! -- fix this, but since it's a fairly clear correctness bug right now (indeed it's holding up ykjit/yklua#124), I think the right thing to do is to revert it (and the subsequent commits that tweak it) and give me the time to fix it at my leisure (if I think it's worth fixing).