- Define the "opcode" data type and reference the Instruction.def file
- Clean up the definition of symtab by indicating that it is composed of
two llists.
- Fix some broken links.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15843 91177308-0d34-0410-b5e6-96231b3b80d8