The x86 vector instruction set is extremely irregular.  With newer
editions, Intel has filled in some of the blanks.  However, we don't
get many 64-bit operations until SSE4.2, introduced in 2009.

The subsequent edition was for AVX1, introduced in 2011, which added
three-operand addressing, and adjusts how all instructions should be
encoded.

Given the relatively narrow 2 year window between possible to support
and desirable to support, and to vastly simplify code maintainence,
I am only planning to support AVX1 and later cpus.

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