Last function call referrer

I suggest to allow us to refer to the last returned result of the function, like this:
```
If Foo(x) <> y Then Return _Foo
```

VB compiler can compile it to:
```
Dim _Foo = Foo(x)
If _Foo <> y Then Return _Foo
```

Where <> y is just a placeholeder for any valid condition.

The _Foo will be generated only when it is used, so this will not affect functions performance (i.e. not all functiomns will have such back-field).

Note that _foo can be used more as needed in its scope:
```
If Foo(x) <> y Then Return _Foo
return _Foo - 1
```