[dev.regabi] cmd/compile: rewrite a few ++/--/+=/-= to prep for getters/setters [generated]

These are trivial rewrites that are only OK because it turns out that n has no side effects.
Separated into a different CL for easy inspection.

[git-generate]
cd src/cmd/compile/internal/gc
rf '
        ex . ../ir ../ssa {
                import "cmd/compile/internal/ir"
                var n *ir.Node
		var i int64

                n.Xoffset++ -> n.Xoffset = n.Xoffset + 1
                n.Xoffset-- -> n.Xoffset = n.Xoffset - 1
                n.Xoffset += i -> n.Xoffset = n.Xoffset + i
                n.Xoffset -= i -> n.Xoffset = n.Xoffset - i
	}
'

Change-Id: If7b4b7f7cbdafeee988e04d03924ef0e1dd867b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/272932


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>