cmd/compile/internal/syntax: avoid follow-up error for incorrect if statement

This is a follow-up on https://go-review.googlesource.com/36470
and leads to a more stable fix. The above CL relied on filtering
of multiple errors on the same line to avoid more than one error
for an `if` statement of the form `if a := 10 {}`. This CL avoids
the secondary error ("missing condition in if statement") in the
first place.

For #18915.

Change-Id: I8517f485cc2305965276c17d8f8797d61ef9e999
Reviewed-on: https://go-review.googlesource.com/36479
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1700,6 +1700,8 @@ func (p *parser) stmtBody(context string) []Stmt {
 	return body
 }
 
+var dummyCond = &Name{Value: "false"}
+
 func (p *parser) header(forStmt bool) (init SimpleStmt, cond Expr, post SimpleStmt) {
 	if p.tok == _Lbrace {
 		return
@@ -1746,12 +1748,8 @@ func (p *parser) header(forStmt bool) (init SimpleStmt, cond Expr, post SimpleSt
 	case *ExprStmt:
 		cond = s.X
 	default:
-		// Not obviously a syntax error but by making it one, we get
-		// automatic filtering of multiple syntax error messages per
-		// line in the compiler. This avoids the follow-up error
-		// "missing condition in if statement" for an if statement
-		// (minimal fix for #18915).
 		p.syntax_error(fmt.Sprintf("%s used as value", String(s)))
+		cond = dummyCond // avoid follow-up error for if statements
 	}
 
 	p.xnest = outer