Move all the byte-sized settings to the front of the byte-vector, and add a
mechanism for assigning numbers to predicates that have no name as well as
predicates from the parent settings group.
This way, all the boolean predicates that are used by a target ISA appear as a
contiguous bit-vector that is a suffix of the settings byte-vector. This
bit-vector can then be indexed linearly when resolving ISA predicates on
encodings.
Add a numbered_predicate() method to the generated Flags structs that can read
a predicate by number dynamically.
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