[mlir][vector] Refactor linearize.mlir (#86648)

This patch refactors the `linearize.mlir` test - currently it contains
some duplication and can be tricky to follow.

Summary of changes:
  * reduce duplication by introducing a shared check prefix (`ALL`) and
    by introducing `-check-prefixes`,
  * make sure that every "check" line is directly above the
    corresponding line of input MLIR,
  * group check lines corresponding to a particular prefix together (so
    that it's easier to see the expected output for a particular
    prefix),
  * remove `CHECK` from prefix names (with multiple prefixes that's just
    noise that can be avoided) and use a bit more descriptive prefixes
    instead (`CHECK0` -> `BW-0`, where `BW` stands for bitwidth),
  * unify indentation,
  * `nonvec_result` -> `test_tensor_no_linearize` (for consistency with
    `test_index_no_linearize`).

NOTE: This change only updates the format of the "CHECK" lines and
doesn't affect what's being tested.

This change is intended as preparation for adding support for scalable
vectors to `LinearizeConstant` and `LinearizeVectorizable` - i.e.
patterns that `linearlize.mlir` is meant to test.