Add DSLX optimization tests for is_pow2

The following DSLX cods should be added to xls/tests/dslx_optimization_test. In general, tests in this file verify that a snippet of DSLX code is properly optimized. The test should verify this library function is properly optimized:

pub fn is_pow2(x: uN[N]) -> bool {
x > uN[N]:0 && (x & (x - uN[N]:1) == uN[N]:0)
}

Also add an equivalent implementation:

pub fn is_pow2(x: uN[N]) -> bool {
let shift = clog2(x);
x > uN[N]:0 && ((x >> shift) << shift == x)
}

Ideally I think this should optimize to something like:

one_hot(x, ls_prio=true) == one_hot(x, lsb_prio=false) && x != 0

Not sure what the sequence of transformations that would get us there though.