JIT: streamline temp usage for returns (#20640)

JIT: streamline temp usage for returns (#20640)

If the jit decides it needs a return spill temp, and the return value
has already been spilled to a single-def temp, re-use the existing
for the return temp rather than creating a new one.

In conjunction with #20553 this allows late devirtualization for calls where
the object in the virtual call is the result of an inline that provides
a better type, and the objected formerly reached the call via one or more
intermediate temps.

Closes #15873.