[RISCV] Combine (select_cc (srl (and X, 1<<C), C), 0, eq/ne, true, fale)

[RISCV] Combine (select_cc (srl (and X, 1<<C), C), 0, eq/ne, true, fale)

(srl (and X, 1<<C), C) is the form we receive for testing bit C.
An earlier combine removed the setcc so it wasn't there to match
when we created the SELECT_CC. This doesn't happen for BR_CC because
generic DAG combine rebuilds the setcc if it is used by BRCOND.

We can shift X left by XLen-1-C to put the bit to be tested in the
MSB, and use a signed compare with 0 to test the MSB.