nv50/ir: it appears that OP_DISCARD can't take a join modifier

nvdisasm does not print a .S even though the bit is set.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp

@@ -2968,6 +2968,7 @@ FlatteningPass::visit(BasicBlock *bb)
          insn = insn->prev;
          if (insn && !insn->getPredicate() &&
              !insn->asFlow() &&
+             insn->op != OP_DISCARD &&
              insn->op != OP_TEXBAR &&
              !isTextureOp(insn->op) && // probably just nve4
              !isSurfaceOp(insn->op) && // not confirmed