Renamed MCInstFragment to MCRelaxableFragment and added some comments.

No change in functionality.

llvm-svn: 171822
diff --git a/llvm/lib/MC/MCELFStreamer.cpp b/llvm/lib/MC/MCELFStreamer.cpp
index e9afff6..b08fa41 100644
--- a/llvm/lib/MC/MCELFStreamer.cpp
+++ b/llvm/lib/MC/MCELFStreamer.cpp
@@ -343,7 +343,7 @@
 
 void MCELFStreamer::EmitInstToFragment(const MCInst &Inst) {
   this->MCObjectStreamer::EmitInstToFragment(Inst);
-  MCInstFragment &F = *cast<MCInstFragment>(getCurrentFragment());
+  MCRelaxableFragment &F = *cast<MCRelaxableFragment>(getCurrentFragment());
 
   for (unsigned i = 0, e = F.getFixups().size(); i != e; ++i)
     fixSymbolsInTLSFixups(F.getFixups()[i].getValue());