Luke Wilde 589c3771e9 LibJS: Only store MemberExpression object when loading a computed prop ...

When calling emit_load_from_reference with a MemberExpression, it is
only necessary to store the result of evaluating MemberExpression's
object when performing computed property lookup.

This allows us to skip unnecessary stores for identifier lookup.
For example, this would generate 3 unnecessary stores:
```
> Temporal.PlainDateTime.prototype.add
JS::Bytecode::Executable (REPL)
1:
[   0] GetVariable 0 (Temporal)
[  28] Store $2
[  30] GetById 1 (PlainDateTime)
[  40] Store $3
[  48] GetById 2 (prototype)
[  58] Store $4
[  60] GetById 3 (add)
```

With this, it generates:
```
> Temporal.PlainDateTime.prototype.add
JS::Bytecode::Executable (REPL)
1:
[   0] GetVariable 0 (Temporal)
[  28] GetById 1 (PlainDateTime)
[  38] GetById 2 (prototype)
[  48] GetById 3 (add)
```

2022-03-28 14:05:33 +02:00

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