R600/SI: Add some mubuf testcases.

I noticed some odd looking cases where addr64 wasn't set
when storing to a pointer in an SGPR. This seems to be intentional,
and partially tested already.

The documentation seems to describe addr64 in terms of which registers
addressing modifiers come from, but I would expect to always need
addr64 when using 64-bit pointers. If no offset is applied,
it makes sense to not need to worry about doing a 64-bit add
for the final address. A small immediate offset can be applied,
so is it OK to not have addr64 set if a carry is necessary when adding
the base pointer in the resource to the offset?