[x86] Rearrange the code for v16i16 lowering a bit for clarity and to
reduce the amount of checking we do here.

The first realization is that only non-crossing cases between 128-bit
lanes are handled by almost the entire function. It makes more sense to
handle the crossing cases first.

THe second is that until we actually are going to generate fancy shared
lowering strategies that use the repeated semantics of the v8i16
lowering, we should waste time checking for repeated masks. It is
simplest to directly test for the entire unpck masks anyways, so we
gained nothing from this.

This also matches the structure of v32i8 more closely.

No functionality changed here.