[CodeGen] Forward-declare TargetMachine (NFC)

InstrEmitter.h needs TargetMachine but relies on a forward declaration
of TargetMachine in MachineOperand.h.  This patch adds a forward
declaration right in InstrEmitter.h.

While we are at it, this patch removes the one in MachineOperand.h,
where it is unnecessary.

Author: kazutakahirata

llvm/include/llvm/CodeGen/MachineOperand.h

@@ -33,7 +33,6 @@ class MachineRegisterInfo;
 class MCCFIInstruction;
 class MDNode;
 class ModuleSlotTracker;
-class TargetMachine;
 class TargetIntrinsicInfo;
 class TargetRegisterInfo;
 class hash_code;

llvm/lib/CodeGen/SelectionDAG/InstrEmitter.h

@@ -26,6 +26,7 @@ class MCInstrDesc;
 class SDDbgLabel;
 class SDDbgValue;
 class TargetLowering;
+class TargetMachine;
 
 class LLVM_LIBRARY_VISIBILITY InstrEmitter {
   MachineFunction *MF;