R600/SI: Use same names for corresponding MUBUF operands and encoding fields

The code emitter knows how to encode operands whose name matches one of
the encoding fields.  If there is no match, the code emitter relies on
the order of the operand and field definitions to determine how operands
should be encoding.  Matching by order makes it easy to accidentally break
the instruction encodings, so we prefer to match by name.

Reviewed-by: Christian König <christian.koenig@amd.com>
llvm-svn: 178930