CodeGen: Use MachineInstr& in SlotIndexes.cpp, NFC

Avoid implicit conversions from iterator to pointer by preferring
MachineInstr& and using range-based for loops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274354 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed