go/printer: reuse go/token.FileSet.PositionFor calls in setPos

setPos is called for most nodes, and in a number of cases,
the position is already the same.
PositionFor is a relatively expensive call,
as it needs to "unpack" a token.Pos into a token.Position.

If we can tell that the position is the same in a cheap way,
we can then avoid calling setPos and PositionFor.
Luckily, we can get the position's offset within the file,
and it doesn't involve the relatively expensive unpacking.

	name          old time/op    new time/op    delta
	PrintFile-16    4.79ms ± 1%    4.36ms ± 1%  -8.88%  (p=0.008 n=5+5)

	name          old speed      new speed      delta
	PrintFile-16  10.8MB/s ± 1%  11.9MB/s ± 1%  +9.73%  (p=0.008 n=5+5)

	name          old alloc/op   new alloc/op   delta
	PrintFile-16     106kB ± 1%     106kB ± 1%    ~     (p=0.167 n=5+5)

	name          old allocs/op  new allocs/op  delta
	PrintFile-16     2.42k ± 0%     2.42k ± 0%    ~     (all equal)

This does assume that the positions of a node being printed are all
within a file, as go/token.Position.Offset is relative to each file.
This seems like a perfectly fine assumption to make right now,
as the largest node which can be printed is an *ast.File.

Change-Id: I2ae55f507ba8ba9f280898c9c8e01c994d9b2a26
Reviewed-on: https://go-review.googlesource.com/c/go/+/461739
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/go/printer/printer.go

@@ -878,8 +878,12 @@ func mayCombine(prev token.Token, next byte) (b bool) {
 }
 
 func (p *printer) setPos(pos token.Pos) {
+	// If p.pos is already equivalent to pos,
+	// we can avoid calling posFor again.
 	if pos.IsValid() {
-		p.pos = p.posFor(pos) // accurate position of next item
+		if file := p.fset.File(pos); file != nil && file.Offset(pos) != p.pos.Offset {
+			p.pos = p.posFor(pos) // accurate position of next item
+		}
 	}
 }