target/arm: Fix do_predset for large VL

Use MAKE_64BIT_MASK instead of open-coding.  Remove an odd
vector size check that is unlikely to be more profitable
than 3 64-bit integer stores.  Correct the iteration for WORD
to avoid writing too much data.

Fixes RISU tests of PTRUE for VL 256.

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: 20180705191929.30773-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>