cmd/compile/internal/ir: tweak a couple names

cmd/compile/internal/ir: tweak a couple names

CallExpr.X -> CallExpr.Fun

This consistent with go/ast and cmd/compile/internal/syntax.

OPRINTN -> OPRINTLN

This op represents the "println" builtin; might as well spell it the
same way.

Change-Id: Iead1b007776658c717879cf0997b3c48028428f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/532795
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>