MC: Add MCInstFragment, not used yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99229 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/MC/MCMachOStreamer.cpp b/lib/MC/MCMachOStreamer.cpp
index b57f302..51a2d45 100644
--- a/lib/MC/MCMachOStreamer.cpp
+++ b/lib/MC/MCMachOStreamer.cpp
@@ -375,6 +375,9 @@
 
   CurSectionData->setHasInstructions(true);
 
+  // FIXME-PERF: Common case is that we don't need to relax, encode directly
+  // onto the data fragments buffers.
+
   SmallVector<MCFixup, 4> Fixups;
   SmallString<256> Code;
   raw_svector_ostream VecOS(Code);