[x86] Switch PAUSE instruction to use XS prefix instead of HasREPPrefix. Remove HasREPPrefix support from disassembler table generator since its now only used by CodeGenOnly instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201767 91177308-0d34-0410-b5e6-96231b3b80d8