[AMDGPU] Disable unused gfx10 dpp instructions

Inhibit generation of unused real dpp instructions on gfx10 just
like it is done on other subtargets. This does not change anything
because these are illegal anyway and not accepted, but it does
reduce the number of instruction definitions generated.

Differential Revision: https://reviews.llvm.org/D68607

llvm-svn: 374083