nv50/ir: optimize shift of 0 bits

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Pierre Moreau <dev@pmoreau.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10164>

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

@@ -1461,6 +1461,12 @@ ConstantFolding::opnd(Instruction *i, ImmediateValue &imm0, int s)
    {
       if (s != 1 || i->src(0).mod != Modifier(0))
          break;
+
+      if (imm0.reg.data.u32 == 0) {
+         i->op = OP_MOV;
+         i->setSrc(1, NULL);
+         break;
+      }
       // try to concatenate shifts
       Instruction *si = i->getSrc(0)->getInsn();
       if (!si)