Reland [X86] X86TTIImpl::getInterleavedMemoryOpCostAVX2(): use getMemoryOpCost()

Now that getMemoryOpCost() correctly handles all the vector variants,
we should no longer hand-roll our own version of it, but use it directly.

The AVX512 variant probably needs a similar change,
but there it is less obvious.

This was initially landed in 69ed93a4,
but was reverted in 6b95fd19
because the patch it depends on was reverted.