tcg: Restrict check_size_impl to multiples of the line size

Normally this is automatic in the size restrictions that are placed
on vector sizes coming from the implementation.  However, for the
legitimate size tuple [oprsz=8, maxsz=32], we need to clear the final
24 bytes of the vector register.  Without this check, do_dup selects
TCG_TYPE_V128 and clears only 16 bytes.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20180705191929.30773-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>