[mlir][nfc] Update vectorize-tensor-extract.mlir (3/N) (#119121)

Tests in "vectorize-tensor-extract.mlir" are inconsistent and would
benefit from refactoring to:

* Clearly categorize tests into "contiguous load," "gather load," and
  "scalar load + broadcast" cases, reflecting the structure of
  tensor.extract vectorization.
* Unify variable naming (both MLIR and FileCheck).
* Ensure all tests exercise unmasked vectorization (masked vectorization
  is covered in "vectorize-tensor-extract-masked.mlir").
* Improve and standardize formatting.

These changes will make it easier to identify the test cases being
exercised and simplify future maintenance or refactoring.

This is patch 3/N in the series. Below is a summary of the changes in
this patch.

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Summary of patch 3/N
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* Cluster all tests for "scalar load + broadcast" together
* Unify MLIR and FileCheck variable names, e.g. `%input`, `%output` ->
  `%src`, `%init`.

Note, I haven't changed test function names to make it easier to track
changes (this PR is mostly about moving code). I will send a seperate PR
to rename the tests.

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Previous patches
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* https://github.com/llvm/llvm-project/pull/118977
* https://github.com/llvm/llvm-project/pull/119080

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