[mlir][nvvm][rocdl] refactor NVVM and ROCDL dialect. NFC.

- Extract common logic between -convert-gpu-to-nvvm and -convert-gpu-to-rocdl.
- Cope with the fact that alloca operates on different addrspaces between NVVM
  and ROCDL.
- Modernize unit tests for ROCDL dialect.

Differential Revision: https://reviews.llvm.org/D79021