[RISCV] Add more SHXADD patterns.

This handles the code we get for this.

int foo(unsigned x, int *y) {
    return y[x >> 3];
}

The srl and shl implied by the array index will be combined to
form (srl (and X, C2), C1). We need to reverse this get to back
the shl to fold into SHXADD.