R600/SI: Report offset in correct units for st64 DS instructions

Need to convert the 64 element offset into bytes, not just the element
size like the normal case instructions.

Noticed by inspection. This can't be hit now because
st64 instructions aren't emitted during instruction selection,
and the post-RA scheduler isn't enabled.
R600/SI: Fix losing chain when fixing reg class of loads.

The lost chain resulting in earlier side effecting nodes
being deleted.