[UnitTests] Add initial set of dedicated early-exit unit tests.

[fhahn]

Adds initial unit tests for early-exit vectorization covering a variation of auto-vectorization and forced interleaving with pragmas.

The interleaving variant is currently mis-compiled and needs

• https://github.com/llvm/llvm-project/pull/145340
• https://github.com/llvm/llvm-project/pull/145394.

We should probably extend the tests to make sure we cover various other scenarios, including returning the loaded element for the early exit, different index types and array sizes.