Replaces uses of unsigned for indexes in LiveInterval and VNInfo with

Replaces uses of unsigned for indexes in LiveInterval and VNInfo with
a new class, MachineInstrIndex, which hides arithmetic details from
most clients. This is a step towards allowing the register allocator
to update/insert code during allocation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81040 91177308-0d34-0410-b5e6-96231b3b80d8