Move GlobalMerge from Transform to CodeGen.

This patch is to move GlobalMerge pass from Transform/Scalar                                                           
to CodeGen, because GlobalMerge depends on TargetMachine.
In the mean time, the macro INITIALIZE_TM_PASS is also moved
to CodeGen/Passes.h. With this fix we can avoid making
libScalarOpts depend on libCodeGen.

llvm-svn: 210951