Reuse data fragments while lowering

include/llvm/MC/MCAsmLayout.h

Index: include/llvm/MC/MCAsmLayout.h
===================================================================
--- include/llvm/MC/MCAsmLayout.h	(revision 119741)
+++ include/llvm/MC/MCAsmLayout.h	(working copy)
@@ -59,9 +59,16 @@
   /// the delta from the old size.
   void UpdateForSlide(MCFragment *F, int SlideAmount);
 
-  /// \brief Update the layout because a fragment has been replaced.
-  void FragmentReplaced(MCFragment *Src, MCFragment *Dst);
+  /// \brief Update the layout, replacing Src with Dst. The contents
+  /// of Src and Dst are not modified, and must be copied by the caller.
+  /// Src will be removed from the layout, but not deleted.
+  void ReplaceFragment(MCFragment *Src, MCFragment *Dst);
 
+  /// \brief Update the layout to coalesce Src into Dst. The contents
+  /// of Src and Dst are not modified, and must be coalesced by the caller.
+  /// Src will be removed from the layout, but not deleted.
+  void CoalesceFragments(MCFragment *Src, MCFragment *Dst);
+
   /// \brief Perform a full layout.
   void LayoutFile();