interp: fix issue where a var is reused instead of redefined

Avoid a spurious optimisation which forces a variable to be reused instead of redefined for assignment operation. This ensures that a variable defined in a loop is re-allocated, preserving the previous instance when used by a closure for example.

Fix #1594

_test/issue-1594.go

@@ -0,0 +1,17 @@
+package main
+
+func main() {
+	var fns []func()
+	for _, v := range []int{1, 2, 3} {
+		x := v*100 + v
+		fns = append(fns, func() { println(x) })
+	}
+	for _, fn := range fns {
+		fn()
+	}
+}
+
+// Output:
+// 101
+// 202
+// 303

interp/cfg.go

@@ -755,7 +755,7 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 					n.gen = nop
 					src.findex = dest.findex
 					src.level = level
-				case len(n.child) < 4 && isArithmeticAction(src) && !isInterface(dest.typ):
+				case len(n.child) < 4 && n.kind != defineStmt && isArithmeticAction(src) && !isInterface(dest.typ):
 					// Optimize single assignments from some arithmetic operations.
 					src.typ = dest.typ
 					src.findex = dest.findex