AMDGPU: Start using !con operator

We have a lot of operand definition work essentially producing
every valid permutation of operands to workaround builiding
operand lists based on the instruction features. Apparently tablegen
already has a mostly undocumented operator to concat dags which
simplies this.

Convert one simple place to use this. The BUF instruction definitions
have much more complicated logic that can be totally rewritten now.

llvm-svn: 312822