[mlir][spirv] Fix a few issues in ModuleCombiner

- Fixed symbol insertion into `symNameToModuleMap`. Insertion
  needs to happen whether symbols are renamed or not.
- Added check for the VCE triple and avoid dropping it.
- Disabled function deduplication. It requires more careful
  rules. Right now it can remove different functions.
- Added tests for symbol rename listener.
- And some other code/comment cleanups.

Reviewed By: ergawy

Differential Revision: https://reviews.llvm.org/D106886