toy windowing auto-vec miss

[Bruce Carneal]

Here's a simple godbolt example of one of the areas in which gdc 
solidly outperforms ldc wrt auto-vectorization: simple but not 
trivial operand gather
https://godbolt.org/z/ox1vvxd8s


Compile time target adaptive manual __vector-ization is an answer 
here if you have no access to SIMT, so not a show stopper, but 
the code is less readable.

I'm not sure what the data parallel future should look like wrt 
language/IR but I'm pretty sure we can do better than praying 
that the auto vectorizer can dig patterns out of for loops, or 
throwing ourselves on the manual vectorization grenade, 
repeatedly.