X86GenInstrInfo size reduction

Replaced the string based 16-bit equivalence check by one based on instruction IDs, thus reducing library size and increasing performance.

This was done by running the original X86GenInstrInfo.inc through another preprocessing step, which executed the original is16BitEquivalent function for all instruction IDs.
Each matching ID gets an entry in the x86_16_bit_eq_tbl table. An instruction may have multiple entries in the table.
x86_16_bit_eq_lookup provides a mapping from an instruction ID to the 16-bit equivalence data. It uses a 1-based index so entries for instructions that do not have an equivalent 16-bit instruction can be set to 0.