Coverity reports out-of-bound accesses within cputlb.c.
This should be a false positive due to how the index is
decoded from MemOpIdx.  To be fair, nothing is checking
the correct bounds during encoding either.

Assert index in range before use, both to catch user errors
and to pacify static analysis.

Fixes: Coverity CID 1487120, 1487127, 1487170, 1487196, 1487215, 1487238
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220401170813.318609-1-richard.henderson@linaro.org>