Current AllowedOpCodes is 1024, which is not enough for modern x86.
Changed limit to 2048 (though AVX512 will exceed this).

Additional Z-cases and ytab tables are added to make it possible
to handle missing AVX1 and AVX2 instructions.

This CL is required by x86avxgen to work properly:
https://go-review.googlesource.com/c/arch/+/66972

Change-Id: I290214bbda554d2cba53349f50dcd34014fe4cee
Reviewed-on: https://go-review.googlesource.com/70650
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>