CodeGen/LLVMTargetMachine: Refactor ISel pass construction; NFCI

- Move ISel (and pre-isel) pass construction into TargetPassConfig
- Extract AsmPrinter construction into a helper function

Putting the ISel code into TargetPassConfig seems a lot more natural and
both changes together make make it easier to build custom pipelines
involving .mir in an upcoming commit. This moves MachineModuleInfo to an
earlier place in the pass pipeline which shouldn't have any effect.

llvm-svn: 304754