[Bug][compiler] compiler crashes when compiling select variants with generics

[rahxephon89]

🐛 Bug

When compiling the following code:

module 0x42::assign {

    public enum E2<T1, T2, T3> has drop {
        A {x: T1},
        B {y: T3, x: T2}
    }
    fun local_get_x2<T2, T2, T3>(e: &E2<T2, T2, T3>): &T2 {
        &e.x
    }
}

compiler will crash with following error:

thread '<unnamed>' panicked at third_party/move/move-model/src/builder/exp_builder.rs:894:53:
index out of bounds: the len is 2 but the index is 2