[avx] the ops that chop 256-bit lanes to 128-bit lanes.

[Lokathor]

The first time through I skipped over these because at first I didn't realize that they do have an inverse version that doesn't have undefined memory. I thought that the only inverses of these ops would just he undefined register content.

then i got to the end of the avx list and saw that you can zero-extend a register's high bits.

So now we want to have these in the lib again.