[mlir][linalg] Consolidate tests for scalable vectorization (#141469)

This patch moves scalable vectorization tests into an existing generic
vectorization test file:
  * vectorization-scalable.mlir --> merged into vectorization.mlir

Rationale:
  * Most tests in vectorization-scalable.mlir are variants of existing
    tests in vectorization.mlir. Keeping them together improves
    maintainability.
  * Consolidating tests makes it easier to spot gaps in coverage for
    regular vectorization.
  * In the Vector dialect, we don't separate tests for scalable vectors;
    this change aligns Linalg with that convention.

Notable changes beyond moving tests:
  * Updated one of the two matrix-vector multiplication tests to use
    `linalg.matvec` instead of `linalg.generic`. CHECK lines remain
    unchanged.
  * Simplified the lone `linalg.index` test by removing an unnecessary
    `tensor.extract`. Also removed canonicalization patterns from the
    TD sequence for consistency with other tests.

This patch contributes to the implementation of #141025 — please refer
to that ticket for full context.

Author: banach-space