Calculate stack offset from array index

tests/expectations/array_with_variable_index.txt

@@ -0,0 +1,2 @@
+5
+7

tests/vm/array_with_variable_index.elna

@@ -0,0 +1,11 @@
+proc main() {
+  var a: array[2] of int;
+  var i: int;
+
+  i := 1;
+  a[0] := 5;
+  a[i] := 7;
+
+  printi(a[0]);
+  printi(a[i]);
+}