[x86] Teach the new vector shuffle lowering the first step toward more
actual support for complex AVX shuffling tricks. We can do independent
blends of the low and high 128-bit lanes of an avx vector, so shuffle
the inputs into place and then do the blend at 256 bits. This will in
many cases remove one blend instruction.

The next step is to permute the low and high halves in-place rather than
extracting them and re-inserting them.
[x86] Fix a helper to reflect that what we actually care about is
128-bit lane crossings, not 'half' crossings. This came up in code
review ages ago, but I hadn't really addresesd it. Also added some
documentation for the helper.

No functionality changed.