[CodeGen] Include MachineInstr.h
This patch teaches DbgEntityHistoryCalculator.h to include
MachineInstr.h so that the header file is self contained.
Without this patch, a C++ source file that only includes
DbgEntityHistoryCalculator.h does not compile because:
PointerIntPair<const MachineInstr *, 1, EntryKind> Instr;
wants to know the alignment of llvm::MachineInstr:
llvm/include/llvm/Support/PointerLikeTypeTraits.h:61:28: error:
invalid application of 'alignof' to an incomplete type
'llvm::MachineInstr'
detail::ConstantLog2<alignof(T)>::value;
^~~~~~~~~~