i965/fs: support doubles with shared variable stores

i965/fs: support doubles with shared variable stores

This is pretty much the same we do with SSBOs.

v2: do not shuffle in-place, it is not safe since the original 64-bit data
    could be used after the write, instead use a temporary like we do
    for SSBO stores (Iago)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>