[llvm-exegesis] Provide a way to handle memory instructions.

Summary:
And implement memory instructions on X86.

This fixes PR36906.

Reviewers: gchatelet

Reviewed By: gchatelet

Subscribers: lebedev.ri, filcab, mgorny, tschuett, RKSimon, llvm-commits

Differential Revision: https://reviews.llvm.org/D48935

llvm-svn: 338567
diff --git a/llvm/tools/llvm-exegesis/lib/Target.cpp b/llvm/tools/llvm-exegesis/lib/Target.cpp
index 44156c8..67a9ea6 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Target.cpp
@@ -30,9 +30,8 @@
     FirstTarget = Target;
     return;
   }
-  assert(Target->Next == nullptr && "target has already been registered");
   if (Target->Next != nullptr)
-    return;
+    return; // Already registered.
   Target->Next = FirstTarget;
   FirstTarget = Target;
 }