[llvm-exegesis] Correclty handle all X86 memory encoding formats.

Summary:
Add unit tests to check the support for each supported format to avoid
regressions such as the one in PR36906.

Reviewers: gchatelet

Subscribers: tschuett, lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D54144

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346330 91177308-0d34-0410-b5e6-96231b3b80d8