[X86] Rewrite `getScalarizationOverhead()`

All of our insert/extract ops work on 128-bit lanes.

For `Insert`, we need to extract affected 128-bit lane,
unless it's being fully overwritten (FIXME: do we need to be
careful about legalization-induced padding that we obviously don't demand?),
perform insertions, and then insert the 128-bit lane back.

But hold on. If we are operating on an 256-bit legal vector,
and thus have two 128-bit subvectors, and are fully overwriting them both,
we don't actually need to insert *both* subvectors,
only the second one, into the implicitly-widened first one.

Also, `Insert` wasn't actually querying the costs,
but just assuming them to be `1`.

`getShuffleCost(TTI::SK_ExtractSubvector)` notes:
```
  // Note that in general, the insertion starting at the beginning of a vector
  // isn't free, because we need to preserve the rest of the wide vector.
```
... so as far as i can tell, we didn't account for that.

I was hoping this would allow vectorization at a higher VF at one case i looked at,
but the subvector insertion cost is still dis-advising that.

The change for `Extract` is NFC, and is for consistency only,
i wanted to get rid of of that weird explicit discounting of insertion of 0'th element,
since the general code should already deal with that.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D137913