[mlir][Vector] add pass option to `convert-vector-to-llvm` to emit opaque pointers

Part of https://discourse.llvm.org/t/rfc-switching-the-llvm-dialect-and-dialect-lowerings-to-opaque-pointers/68179

This patch allows the use of `convert-vector-to-llvm` with LLVM opaque pointer types. The changes required were relatively minor:
* there were a few GEPs and loads that required specifying the result or base type
* a few bitcasts, especially when interacting with memref have to not be emitted when using opaque pointers

Differential Revision: https://reviews.llvm.org/D143853