Reworked the Intel disassembler to support instructions

whose opcodes extend into the ModR/M field using the
Form field of the instruction rather than by special
casing each instruction.  Commented out the special
casing of VMCALL, which is the first instruction to use
this special form.  While I was in the neighborhood,
added a few comments for people modifying the Intel
disassembler. 

llvm-svn: 96043