[x86] put nops into the WriteNop class and customize for Jaguar

1. Given that we already have a classification bucket with 'nop' in the name, 
   that's where 'nop' belongs. Right now, it's only used for prefix bytes and 'pause'.
2. Make the latency of this class '1' for Jaguar to tell the scheduler (and presumably 
   llvm-mca) how to model the resource requirements better even though a nop has no 
   dependencies.

Differential Revision: https://reviews.llvm.org/D44608

llvm-svn: 327853