bitops.hh/log2ceil(): add special case for n == 1
bitops.hh/log2ceil(): add special case for n == 1

The current implementation will lead to invalid input passed to the
builtin `clz()` when n is 1. To avoid this add a special case for n == 1
and return the precomputed value for it.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <97a95fea2d44a63390e42878948e926e2a4d04fa.1528793345.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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