[instcombine] Canonicalize constant index type to i64 for extractelement/insertelement

The basic idea to this is that a) having a single canonical type makes CSE easier, and b) many of our transforms are inconsistent about which types we end up with based on visit order.

I'm restricting this to constants as for non-constants, we'd have to decide whether the simplicity was worth extra instructions. For constants, there are no extra instructions.

We chose the canonical type as i64 arbitrarily.  We might consider changing this to something else in the future if we have cause.

Differential Revision: https://reviews.llvm.org/D115387