Make pnacl-bccompress add abbreviations for obvious constants.

include/llvm/Bitcode/NaCl/NaClBitcodeDist.h

Index: include/llvm/Bitcode/NaCl/NaClBitcodeDist.h
diff --git a/include/llvm/Bitcode/NaCl/NaClBitcodeDist.h b/include/llvm/Bitcode/NaCl/NaClBitcodeDist.h
index b75fac320138362b4b93d51dc2e8a9ecf98fe67b..54b16a4cac361f0584ae5118eeb628704d60bd03 100644
--- a/include/llvm/Bitcode/NaCl/NaClBitcodeDist.h
+++ b/include/llvm/Bitcode/NaCl/NaClBitcodeDist.h
@@ -312,7 +312,7 @@ public:
   /// Builds the distribution associated with the distribution map.
   /// Warning: The distribution is cached, and hence, only valid while
   /// it's contents is not changed.
-  Distribution *GetDistribution() const {
+  const Distribution *GetDistribution() const {
     if (CachedDistribution == 0) Sort();
     return CachedDistribution;
   }