R600/SI: Use immediates offsets for SMRD instructions whenever possible

There was a problem with the old pattern, so we were copying some
larger immediates into registers when we could have been encoding
them in the instruction.
R600/SI: Add a MUBUF load pattern for Reg+Imm offsets
R600/SI: Add a MUBUF store pattern for Imm offsets
R600/SI: Add a MUBUF store pattern for Reg+Imm offsets