MachineFunction: Store more specific reference to LLVMTargetMachine; NFC

MachineFunction can only be used in code using lib/CodeGen, hence we
can keep a more specific reference to LLVMTargetMachine rather than just
TargetMachine around.

Do the same for references in ScheduleDAG and RegUsageInfoCollector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346183 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed