cmd/compile: retain source positions of arguments to inlined calls

Arguments to inlined calls are hidden from setPos as follows:

    args := as.Rlist
    as.Rlist.Set(nil)
    // setPos...
    as.Rlist.Set(args.Slice())

Previously, this code had no effect since the value of as was
overwritten by the assignment in the retvars loop.

Fixes #19799.

Change-Id: Iaf97259f82fdba8b236136337cc42b2774c7fef5
Reviewed-on: https://go-review.googlesource.com/44351
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/gc/inl.go b/src/cmd/compile/internal/gc/inl.go
index 54317d9..dfa13e3 100644
--- a/src/cmd/compile/internal/gc/inl.go
+++ b/src/cmd/compile/internal/gc/inl.go
@@ -726,9 +726,9 @@
 
 	// Zero the return parameters.
 	for _, n := range retvars {
-		as = nod(OAS, n, nil)
-		as = typecheck(as, Etop)
-		ninit.Append(as)
+		ras := nod(OAS, n, nil)
+		ras = typecheck(ras, Etop)
+		ninit.Append(ras)
 	}
 
 	retlabel := autolabel(".i")