[X86][CLFLUSH]: Adding full coverage of MC encoding for the CLFLUSH isa sets.<NFC>

NFC.
Adding MC regressions tests to cover the CLFLSH and CLFLUSHOPT isa sets.
This patch is part of a larger task to cover MC encoding of all X86 isa sets started in revision: https://reviews.llvm.org/D39952

Reviewers: zvi, RKSimon, craig.topper, m_zuckerman
Differential Revision: https://reviews.llvm.org/D41331

Change-Id: Ifa643dd52f1b7184c52bc1806038dc74b234fc65
llvm-svn: 321153