cmd/compile/internal/syntax: simplify BOM handling

Change-Id: I5e2522d34720d7687e1e9ff0831936702976a1c7

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

@@ -112,8 +112,10 @@ redo:
 
 	// BOM's are only allowed as the first character in a file
 	const BOM = 0xfeff
-	if r == BOM && s.r0 > 0 { // s.r0 is always > 0 after 1st character (fill will set it to 1)
-		s.error("invalid BOM in the middle of the file")
+	if r == BOM {
+		if s.r0 > 0 { // s.r0 is always > 0 after 1st character (fill will set it to 1)
+			s.error("invalid BOM in the middle of the file")
+		}
 		goto redo
 	}
 

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

@@ -46,11 +46,6 @@ func Read(src io.Reader, errh ErrorHandler, mode Mode) (*File, error) {
 	var p parser
 	p.init(src, errh)
 
-	// skip initial BOM if present
-	if p.getr() != '\ufeff' {
-		p.ungetr()
-	}
-
 	p.next()
 	ast := p.file()