Shrink the asm matcher tables.

- Shrink the opcode field to 16 bits.
- Shrink the AsmVariantID field to 8 bits.
- Store the mnemonic string in a string table, store a 16 bit index.
- Store a pascal-style length byte in the string instead of a null terminator,
  so we can avoid calling strlen on every entry we visit during mnemonic search.

Shrinks X86AsmParser.o from 434k to 201k on x86_64 and eliminates relocs from the table.

llvm-svn: 151984