[RISCV] Custom legalize i32 SMULO with RV64LegalI32.

The default lowering will use shifts to make use of an i32 setcc.
We don't support i32 setcc, so its better to sig extend the low
32 bits and compare the full 64 bit result. This gives produces
mul+mulw+xor+snez like we do without RV64LegalI32.