CodeGen: Use MachineInstr& in LocalStackSlotAllocation, NFC

Avoid a number of implicit conversions from iterator to pointer by using
range-based for and MachineInstr&.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274298 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed