Fix UB in 128 bit cttz/ctlz intrinsics

[FractalFir]

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 bit ctlz/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 integer
let 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 integer
let 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.

[antoyo]

Thanks for sending this PR.

How does the generated code compare to the fix made in this PR?

[antoyo]

Done in #635.