Renamed MCInstFragment to MCRelaxableFragment and added some comments.

No change in functionality.

llvm-svn: 171822
diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
index c078794..26df966 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp
@@ -213,7 +213,7 @@
   /// fixup requires the associated instruction to be relaxed.
   bool fixupNeedsRelaxation(const MCFixup &Fixup,
                             uint64_t Value,
-                            const MCInstFragment *DF,
+                            const MCRelaxableFragment *DF,
                             const MCAsmLayout &Layout) const {
     // FIXME.
     assert(0 && "RelaxInstruction() unimplemented");