Optimize router opcode initialization for fallback and text receivers #3330

Currently, the router function separately initializes the op variable and then conditionally loads its value based on the incoming message length whenever fallback or text receivers are present, introducing unnecessary overhead.

Optimization task:
Replace this pattern (used for fallback and text receivers):

int op = 0;
int in_msg_length = slice_bits(in_msg);
if (in_msg_length >= 32) {
    op = in_msg~load_uint(32);
}

With the following optimized conditional assignment:

int in_msg_length = slice_bits(in_msg);
int op = (in_msg_length >= 32) ? in_msg~load_uint(32) : 0;