`testbench1.cpp`: undefined behavior in signed saturation reference code for testcases 610 and 612

[oltolm]

The reference implementation for compress_saturated in testbench1.cpp is incorrect for signed destination types in testcases 610 and 612.

It computes the saturated result with:

(RT(1) << (sizeof(RT) * 8 - 1)) - (elements1[i] > 0)

This has undefined behavior for signed RT because it left-shifts into the sign bit, and for positive overflow cases it can overflow again when subtracting 1.

As a result, the testbench reference code can produce incorrect expected results even when compress_saturated itself is correct.

Expected behavior

For signed narrowing saturation, the reference code should clamp to std::numeric_limits<RT>::max() for positive overflow and std::numeric_limits<RT>::min() for negative overflow.