Speculatively revert commit 127478 (jsjodin) in an attempt to fix the

llvm-gcc-i386-linux-selfhost and llvm-x86_64-linux-checks buildbots.
The original log entry:
Remove optimization emitting a reference insted of label difference, since
it can create more relocations. Removed isBaseAddressKnownZero method,
because it is no longer used.

llvm-svn: 127540