Fixes#604.

This PR changes a few things.

First of all, it enables UB checks for tests in debug mode: This should allow us to catch any regressions.

Second, I extracted the common implementation of the "safe"ctlz/cttz intrinsics into a separate function.

This makes handling the 128 bit edge case a bit easier, and allows us to use those intrinsics.

The third change modifies the 128 bitctlz/cttz emulation to use the "safe" version of the 64 bit intrinsic, instead of calling the GCC built-in directly.

Effectively, this turns a piece of IR like this:

// Get the ctlz for the low and high half of the 128 bit integerlet ctlz_low =ctlz_nonzero(low);let ctlz_high =ctlz_nonzero(high);

Into a piece of code like this:

// Get the ctlz for the low and high half of the 128 bit integerlet ctlz_low =if low !=0{ctlz_nonzero(low)}else{64};let ctlz_high =if high !=0{ctlz_nonzero(high)}else{64};

This solution is not the prettiest(maybe a few less branches could be used?), but it is UB-free, and relatively easy to implement.