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

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>