cmd/compile: spill output parameters passed in registers as autos

ALSO:
found evidence that stack maps for bodyless methods are wrong.
gofmt in test/abi
removed never-executed code in types/size.go

Updates #44816.

Change-Id: I658c33f049337fb6f1e625f0c55430d25bfa877e
Reviewed-on: https://go-review.googlesource.com/c/go/+/300749


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>