Adds support for the GFNI "Galois Field New Instructions" instruction set.
These instructions are not included in the Opcodes database, therefore they're
added using the "extras" mechanism introduced in #345.
For simplicity, the loading phase is updated slightly so that AVX-512 form
expansion rules are applied after extras are added to the list. This greatly
reduces the number of forms that have to be specified by hand.
Based on #343Fixes#335
Co-authored-by: Klaus Post <klauspost@gmail.com>
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