stg_IND[_STATIC] should inspect the indirectees pointer tag before jumping into the indirectees entry code.

Currently this is implemented as:

INFO_TABLE(stg_IND,1,0,IND,"IND","IND")
{
    TICK_ENT_DYN_IND(); /* tick */
    R1 = UNTAG(StgInd_indirectee(R1));
    TICK_ENT_VIA_NODE();
    jump %GET_ENTRY(R1) [R1];
}

I think we should try if this implementation:

{
    W_ tag;
    TICK_ENT_DYN_IND(); /* tick */
    R1 = (StgInd_indirectee(R1));
    tag = GETTAG(R1);
    if(tag == 0) {
        // R1 = UNTAG(R1);

        TICK_ENT_VIA_NODE();
        jump %GET_ENTRY(R1) [R1];
    } else {
        jump W_[Sp](R1);
    }
}

Why so? It's my understanding we sometimes we end up in situations where we have:

x -----> THUNK
y --/

We evaluate x, which leaves us with this relevant part:

y --> IND -[tagged]-> CON

Now if we evaluate y it will jump into the entry code for CON which is just a waste of time. It should rather just jump to the continuation.

---

I had tried this implementation locally and it seemed to work fine and reduce overall instructions executed. But I (or someone else!) needs to properly benchmark this before we make this change.