[mlir][llvm] Switch rountrip tests to opaque pointers.

The revision switches all remaining LLVM dialect roundtrip
tests to opaque pointers. Selected tests are copied to a
postfixed test file for the time being.

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

Reviewed By: zero9178

Differential Revision: https://reviews.llvm.org/D146639