[opt] Initialize WriteBitcode pass.

Probably due to a change of how some pass initializes its dependencies,
the -write-bitcode pass (Bitcode/Writer/BitcodeWriterPass.cpp) is not
initialized in opt anymore and therefore not usable with

opt -write-bitcode

Explicitly call initializeWriteBitcodePassPass() to make it available
in opt again.

Differential Revision: https://reviews.llvm.org/D39223

llvm-svn: 316464

tools/opt/opt.cpp

@@ -405,6 +405,7 @@ int main(int argc, char **argv) {
   initializeCountingFunctionInserterPass(Registry);
   initializeUnreachableBlockElimLegacyPassPass(Registry);
   initializeExpandReductionsPass(Registry);
+  initializeWriteBitcodePassPass(Registry);
 
 #ifdef LINK_POLLY_INTO_TOOLS
   polly::initializePollyPasses(Registry);