Renamed MCInstFragment to MCRelaxableFragment and added some comments.

No change in functionality.

llvm-svn: 171822
diff --git a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
index f9a1ebf..f24edf6 100644
--- a/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
+++ b/llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp
@@ -119,7 +119,7 @@
 
   bool fixupNeedsRelaxation(const MCFixup &Fixup,
                             uint64_t Value,
-                            const MCInstFragment *DF,
+                            const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const {
     // FIXME.
     llvm_unreachable("relaxInstruction() unimplemented");