nv50/ir: clear subop when folding constant expressions

Some operations (e.g. OP_MUL/OP_MAD/OP_EXTBF) might have a subop set.
After folding, make sure that it is cleared

Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>

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

@@ -563,6 +563,7 @@ ConstantFolding::expr(Instruction *i,
    } else {
       i->op = i->saturate ? OP_SAT : OP_MOV; /* SAT handled by unary() */
    }
+   i->subOp = 0;
 }
 
 void