Let Expression Binding Seeing own pattern

[mattbierner]

Currently the evaluation:

var x; let x = x * 3 in x;

produces undefined * 3;

since the generated code is:

var x;
var x0 = x0 * 3;
x0;

Although this is useful when the bound value is a function, this is incorrect in other cases. The generated code should be:

var x;
var x0 = x * 3;
x0;