[TableGen] Optimize the regex search.

llvm::Regex is still the slowest regex engine on earth, running it over
all instructions on X86 takes a while. Extract a prefix and use a binary
search to reduce the search space before we resort to regex matching.

There are a couple of caveats here:
- The generic opcodes are outside of the sorted enum. They're handled in an extra loop.
- If there's a top-level bar we can't use the prefix trick.
- We bail on top-level ?. This could be handled, but it's rare.

This brings the time to generate X86GenInstrInfo.inc from 21s to 4.7s on
my machine.

llvm-svn: 323277