[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