MC: Add TargetAsmBackend::MayNeedRelaxation, for checking whether a particular instruction + fixups might need relaxation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99249 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index 7672528..031820d 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -715,7 +715,7 @@
 
       // Create a new data fragment for the instruction.
       //
-      // FIXME: Reuse previous data fragment if possible.
+      // FIXME-PERF: Reuse previous data fragment if possible.
       MCDataFragment *DF = new MCDataFragment();
       SD.getFragmentList().insert(it2, DF);