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

Repair of CL 300749.

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.
Updates #40724.

Change-Id: Ifeb5fee60f60e7c7b58ee0457f58a3265d6cf3f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/302071


Trust: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>