Currently, when determining which concrete operation an instruction
uses, Fuzzilli uses the following code pattern:

    switch instr.op {
        case is LoadInteger:
            ...
        case let op as CallMethod:
            ...
    }

However, this is inefficient as it will be compiled to code such as:

    if (dynamicTypeCast(instr.op, LoadInteger)) {
        // it's a LoadInteger operation
    } else if (dynamicTypeCast(instr.op, CallMethod)) {
        // it's a CallMethod operation
    } else if ...

and therefore require roughly linear time (in the number of operations).

With this change, there is now a new Opcode enum which allows these
switches to be rewritten as:

    switch instr.op.opcode {
        case .loadInteger:
            ...
        case .callMethod(let op):
            ...
    }

The code genererated for this switch is efficient as it uses the enum's
integer value to index into a jumptable.

On a simple benchmark that generates random programs, then lifts them to
JavaScript, this change appears to result in a ~1.5x speedup.