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.

llvm-svn: 346183
diff --git a/llvm/lib/CodeGen/RegUsageInfoCollector.cpp b/llvm/lib/CodeGen/RegUsageInfoCollector.cpp
index 9db2af9..66c7c5c 100644
--- a/llvm/lib/CodeGen/RegUsageInfoCollector.cpp
+++ b/llvm/lib/CodeGen/RegUsageInfoCollector.cpp
@@ -81,7 +81,7 @@
 bool RegUsageInfoCollector::runOnMachineFunction(MachineFunction &MF) {
   MachineRegisterInfo *MRI = &MF.getRegInfo();
   const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
-  const TargetMachine &TM = MF.getTarget();
+  const LLVMTargetMachine &TM = MF.getTarget();
 
   LLVM_DEBUG(dbgs() << " -------------------- " << getPassName()
                     << " -------------------- \n");