[CodeGenPrepare] Make TargetPassConfig required

The code paths in the absence of TargetMachine, TargetLowering or
TargetRegisterInfo are poorly tested. As rL285987 said, requiring
TargetPassConfig allows us to delete many (untested) checks littered
everywhere.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D73754