R600: Promote i64 stores to v2i32

Now we need only one 64-bit pattern for stores.
R600/SI: Use a ComplexPattern for MUBUF stores

Now that non-leaf ComplexPatterns are allowed we can fold all the MUBUF
store patterns into the instruction definition.  We will also be able to
reuse this new ComplexPattern for MUBUF loads and atomic operations.