nv50/ir: OP_JOIN is a flow instruction

OP_JOIN instructions are assumed to be flow instructions and mercilessly
casted to FlowInstruction.

This patch fixes an instance where an OP_JOIN is created as a plain
instruction. This can cause crashes in the ir printer.

[imirkin: add ->fixed = 1]
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nv50.cpp

@@ -887,7 +887,7 @@ NV50LoweringPreSSA::handleTXL(TexInstruction *i)
       }
    }
    bld.setPosition(joinBB, false);
-   bld.mkOp(OP_JOIN, TYPE_NONE, NULL);
+   bld.mkFlow(OP_JOIN, NULL, CC_ALWAYS, NULL)->fixed = 1;
    return true;
 }