cmd/compile/internal/syntax: don't record semi position if there's none

Fixes #23406.

Change-Id: Ief04e20357c9ca03a5e496f1742428394c8ee658
Reviewed-on: https://go-review.googlesource.com/87317
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/parser.go

@@ -1768,9 +1768,13 @@ func (p *parser) header(keyword token) (init SimpleStmt, cond Expr, post SimpleS
 		lit string // valid if pos.IsKnown()
 	}
 	if p.tok != _Lbrace {
-		semi.pos = p.pos()
-		semi.lit = p.lit
-		p.want(_Semi)
+		if p.tok == _Semi {
+			semi.pos = p.pos()
+			semi.lit = p.lit
+			p.next()
+		} else {
+			p.want(_Semi)
+		}
 		if keyword == _For {
 			if p.tok != _Semi {
 				if p.tok == _Lbrace {