* Try and assign directly to return registers; backtrack to use struct-return param
Rather than trying to count number of return registers that would be used by a
given set of return values, optimistically assign the return values to
registers. If we later find that we can't fit them all in registers, then
backtrack and introduce the use of a struct-return pointer parameter.
* Rename `rets2` and wrap it in an option so we avoid the clone for non-multi-value
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