SPV_KHR_cooperative_matrix: clarify requirements on Stride

[kpet]

Fixes #279

[kpet]

We did discuss potentially requiring the stride be such that no write collisions occur. Do we want to do this too?

[bashbaug]

Discussed in the October 8th teleconference:

• Why does constraint only apply to stores? Suggest updating the constraint so it applies more broadly such that no write collisions occur, which should also help to describe why the constraint does not apply to loads. Figuring out the right spec text to describe this is challenging - suggestions are welcome.
• What does it mean to say "interpreted as unsigned"? In short, this means that an integer stride with no signed-ness information is interpreted as an unsigned value, so if the upper bit is set it is interpreted as a very large positive value and not a negative value.
• We're aiming to update the constraint before merging.