[dev.unified] cmd/compile: implicit conversions for return statements

[dev.unified] cmd/compile: implicit conversions for return statements

This CL inserts implicit conversions for simple N:N return
statements. A subsequent CL will handle N:1 return statements.

Change-Id: Ia672db3e214025510485e17d3d50d42ff01bc74e
Reviewed-on: https://go-review.googlesource.com/c/go/+/413517
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>