cmd/gc: fix conversion of runtime constant

The code cannot have worked before, because it was
trying to use the old value in a range check for the new
type, which might have a different representation
(hence the 'internal compiler error').

Fixes #8073.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/98630045

src/cmd/gc/const.c

@@ -951,6 +951,7 @@ unary:
 	case TUP(OCONV, CTFLT):
 	case TUP(OCONV, CTSTR):
 		convlit1(&nl, n->type, 1);
+		v = nl->val;
 		break;
 
 	case TUP(OPLUS, CTINT):

test/fixedbugs/issue8073.go

@@ -0,0 +1,15 @@
+// compile
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// issue 8073.
+// was "internal compiler error: overflow: float64 integer constant"
+
+package main
+
+func main() {
+	var x int
+	_ = float64(x * 0)
+}