But did you think about using a perfect hash function and table? Based on my prior research, it seems like they are almost universally faster on small strings than trees and tries due to lower cache miss rates.
You could copy the instruction to a 16 byte sized buffer and hash the one/two int64s. Looking at the code sample in the article, there wasn't a single instruction longer than 5 characters, and I suspect that in general instructions with short names are more common than those with long names.
This last fact might actually support the current model, as it grows linearly-ish in the size of the instruction, instead of being constant like hash.