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

[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