gm107/ir: add a missing assertion in emitISCADD()

For consistency, similar to the other emitters.

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

src/gallium/drivers/nouveau/codegen/nv50_ir_emit_gm107.cpp

@@ -1858,6 +1858,8 @@ CodeEmitterGM107::emitIMAD()
 void
 CodeEmitterGM107::emitISCADD()
 {
+   assert(insn->src(1).get()->asImm());
+
    switch (insn->src(2).getFile()) {
    case FILE_GPR:
       emitInsn(0x5c180000);