[FIX] qweb: do not leak values in global context in rare cases

In some very rare cases (such as the use of the t-foreach directive),
Owl did leak the values in the render context in the global context.
This was due to the fact that the compiled template looked like this:

let _3 = _4 = _5;

instead of

let _3 = _5;
let _4 = _5;

tests/qweb/__snapshots__/qweb_memory.test.ts.snap

@@ -0,0 +1,37 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`memory t-foreach does not leak stuff in global scope 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let scope = Object.create(context);
+    let h = this.h;
+    let c1 = [], p1 = {key:1};
+    let vn1 = h('p', p1, c1);
+    let _2 = [3,2,1];
+    if (!_2) { throw new Error('QWeb error: Invalid loop expression')}
+    let _3 = _2;
+    let _4 = _2;
+    if (!(_2 instanceof Array)) {
+        _3 = Object.keys(_2);
+        _4 = Object.values(_2);
+    }
+    let _length3 = _3.length;
+    let _origScope5 = scope;
+    scope = Object.create(scope);
+    for (let i1 = 0; i1 < _length3; i1++) {
+        scope.item_first = i1 === 0
+        scope.item_last = i1 === _length3 - 1
+        scope.item_index = i1
+        scope.item = _3[i1]
+        scope.item_value = _4[i1]
+        let key1 = i1;
+        let _6 = scope['item'];
+        if (_6 != null) {
+            c1.push({text: _6});
+        }
+    }
+    scope = _origScope5;
+    return vn1;
+}"
+`;