Allow `@ModifyVariable` to capture locals

[ctrlaltmilk]

I have a function with the structure

void foo() {
    int a = bar();
    int b = baz();

    if (a == 0) {
        doSomethingNontrivial();
    }
}

I would like to use Mixins to rewrite the function a bit:

void foo() {
    int a = bar();
    int b = baz();

    if (a == 0 || b == 0) {
        baz();
    }
}

To do this, the best option seems to be a @ModifyVariable on the value of a, but to do that I would need to capture the value of b as well. Currently, this is not possible. If the a == 0 was a method call, it would be possible to use an @Inject just before the comparison and set a to 0 if b is 0, but as the comparison becomes an IF_ICMPEQ it's impossible to target for an injection. @ModifyVariable already examines the LVT to determine the accesses it needs to inject bytecode at, so adding another local capture shouldn't make it any more fragile than it already is.

[LlamaLad7]

Consider using https://github.com/LlamaLad7/MixinExtras/wiki/Local in the meantime.