tcg/i386: Expand vector word rotate as avx512vbmi2 shift-double

While there are no specific 16-bit rotate instructions, there
are double-word shifts, which can perform the same operation.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>