[mlir][vector] Categorize Vector-to-LLVM conversion tests (nfc) (#125918)

Following #124308, this patch
reorganizes the `vector-to-llvm.mlir` tests by splitting them into two
categories:

- **Basic conversion tests**: Tests that only require
  `populateVectorToLLVMConversionPatterns`, focusing on the minimal
  conversion from Vector to LLVM. These have been moved to
  `vector-to-llvm-interface.mlir`.
- **Full pass tests**: Tests that require the complete
  `ConvertVectorToLLVMPass`, which includes
  `populateVectorToLLVMConversionPatterns` along with additional
  patterns. These remain in `vector-to-llvm.mlir`.

This reorganization clarifies test coverage and helps avoid unnecessary
duplication.

NOTE: This is merely moving tests around between two files and adds
some comments.

Author: banach-space