Correct implementation of wi::clz

Correct implementation of wi::clz

As diagnosed with Jakub and Richard in the analysis of PR 102134, the
current implementation of wi::clz has incorrect/inconsistent behaviour.
As mentioned by Richard in comment #7, clz should (always) return zero
for negative values, but the current implementation can only return 0
when precision is a multiple of HOST_BITS_PER_WIDE_INT.  The fix is
simply to reorder/shuffle the existing tests.

2021-09-06  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* wide-int.cc (wi::clz): Reorder tests to ensure the result
	is zero for all negative values.