cmd/compile: remove work-arounds for 0o/0O octals

With math/big supporting the new octal prefixes directly,
the compiler doesn't have to manually convert such numbers
into old-style 0-prefix octals anymore.

Updates #12711.

Change-Id: I300bdd095836595426a1478d68da179f39e5531a
Reviewed-on: https://go-review.googlesource.com/c/go/+/165861Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/mpfloat.go

@@ -188,28 +188,11 @@ func (a *Mpflt) SetString(as string) {
 		as = as[1:]
 	}
 
-	// Currently, Val.Parse below (== math/big.Float.Parse) does not
-	// handle the 0o-octal prefix which can appear with octal integers
-	// with 'i' suffix, which end up here as imaginary components of
-	// complex numbers. Handle explicitly for now.
-	// TODO(gri) remove once Float.Parse can handle octals (it handles 0b/0B)
-	var f *big.Float
-	if strings.HasPrefix(as, "0o") || strings.HasPrefix(as, "0O") {
-		x, ok := new(big.Int).SetString(as[2:], 8)
-		if !ok {
-			yyerror("malformed constant: %s", as)
-			a.Val.SetFloat64(0)
-			return
-		}
-		f = a.Val.SetInt(x)
-	} else {
-		var err error
-		f, _, err = a.Val.Parse(as, 0)
-		if err != nil {
-			yyerror("malformed constant: %s (%v)", as, err)
-			a.Val.SetFloat64(0)
-			return
-		}
+	f, _, err := a.Val.Parse(as, 0)
+	if err != nil {
+		yyerror("malformed constant: %s (%v)", as, err)
+		a.Val.SetFloat64(0)
+		return
 	}
 
 	if f.IsInf() {

src/cmd/compile/internal/gc/mpint.go

@@ -282,11 +282,8 @@ func (a *Mpint) SetInt64(c int64) {
 }
 
 func (a *Mpint) SetString(as string) {
-	// TODO(gri) remove this code once math/big.Int.SetString can handle 0o-octals and separators
+	// TODO(gri) remove this code once math/big.Int.SetString can handle separators
 	as = strings.Replace(as, "_", "", -1) // strip separators
-	if len(as) >= 2 && as[0] == '0' && (as[1] == 'o' || as[1] == 'O') {
-		as = "0" + as[2:]
-	}
 
 	_, ok := a.Val.SetString(as, 0)
 	if !ok {