cmd/compile: remove AuxCall.results, cleanup ssagen/ssa.go

cmd/compile: remove AuxCall.results, cleanup ssagen/ssa.go

More cleanup to remove unnecessary parts of AuxCall.
Passed testing on arm64 (a link-register architecture)
in addition to amd64 so very likely okay.

(Gratuitously updated commit message to see if it will
correctly this time.)

Updates #40724

Change-Id: Iaece952ceb5066149a5d32aaa14b36755f26bb8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/303433
Trust: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>