Some initial notes on the effects of vldN and vstN vectorisation

I've now submitted the initial vldN and vstN work, so I thought I'd see how often it triggers for natty's libav package. I've put some initial results here:

https://wiki.linaro.org/RichardSandiford/Sandbox/NeonLibAv

There are more files to go through, so this isn't complete. I've also left out cases that were very similar to the ones given.

Some of the code is reasonable, while others are obviously not as good as they could be. I don't think the problems are really to do with the vldN and vstN work itself though. They seem to be due to the underlying interleaved load/store detection, or in the handling of widening operations.

Richard