cmd/compile: do not allocate space for unspilled in-register results

For function results, if in register, we allocate spill slots
within the frame like locals. Currently, even if we never spill
to it the slot is still allocated. This CL makes it not allocate
the slot if it is never used.

Change-Id: Idbd4e3096cfac6d2bdfb501d8efde48ee2191d7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/309150


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