The copy/spill/fill instructions will be used by the register allocator
for splitting live ranges. The copy instruction is also useful when
rewriting values:
If a primary value is rewritten as a secondary result, a copy
instruction can be used instead:
a = foo x
=>
t, vx1 = call ...
a = copy vx1
Since a primary value must be the first value of an instruction, this
doesn't work:
a = foo x
=>
t, a = call ...
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