[mlir][vector] Insert/extract element can accept index

`vector::InsertElementOp` and `vector::ExtractElementOp` have had their `position`
operand changed to accept `AnySignlessIntegerOrIndex` for better operability with
operations that use `index`, such as affine loops.

LLVM's `extractelement` and `insertelement` can also accept `i64`, so lowering
directly to these operations without explicitly inserting casts is allowed. SPIRV's
equivalent ops can also accept `i64`.

Reviewed By: nicolasvasilache, jpienaar

Differential Revision: https://reviews.llvm.org/D114139