[dev.regabi] cmd/compile: unexport ir.FmtNode

It's only used inside package ir now.

[git-generate]

cd src/cmd/compile/internal/ir
rf 'mv FmtNode fmtNode'
sed -i 's/FmtNode/fmtNode/g' mknode.go
go generate

Change-Id: Ib8f6c6984905a4d4cfca1b23972a39c5ea30ff42
Reviewed-on: https://go-review.googlesource.com/c/go/+/279451


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>