avoid var declaration overwritten

[magic-akari]

• fix Regression in v0.14.x #2080 regression introduced in 4fa3d7a

For the following input code

console.log(1, f);

function f() {
  x();
}

console.log(2, f);

var f = 1;

console.log(3, f);

function f() {
  y();
}

console.log(4, f);

var f = 2;

console.log(5, f);

The following output will be generated

1 ƒ f() {
  y();
}
2 ƒ f() {
  y();
}
3 1
4 1
5 2

This means that input is equivalent to the following code

var f;
function f() {
  x();
}
function f() {
  y();
}

console.log(1, f);
console.log(2, f);

f = 1;

console.log(3, f);
console.log(4, f);

f = 2;

console.log(5, f);

Functions can always be safely overwritten, whereas var declaration is not.

This PR tries to preserve the var declaration.

[magic-akari]

It means we should keep all the vars and the last functions.
The equivalent code is as follows.

console.log(1, f);

// Safely delete
// function f() {
//   x();
// }

console.log(2, f);

var f = 1;

console.log(3, f);

// It should be kept since hoisting
function f() {
  y();
}

console.log(4, f);

var f = 2;

console.log(5, f);

[evanw]

Thanks for the PR! When I took a look at this, the bug had a simpler fix that also addressed the underlying issue, so I decided to make that change instead.