[llvm] [AArch64] More accurately model cost of partial reductions (PR #181707)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-mlir-rhel-clang` running on `ppc64le-mlir-rhel-test` while building `llvm` at step 6 "test-build-check-mlir-build-only-check-mlir".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/129/builds/41624

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-check-mlir-build-only-check-mlir) failure: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
...
PASS: MLIR :: mlir-runner/simple.mlir (3990 of 4000)
PASS: MLIR :: Transforms/inlining.mlir (3991 of 4000)
PASS: MLIR :: Dialect/SCF/loop-unroll.mlir (3992 of 4000)
PASS: MLIR :: Transforms/test-operation-folder.mlir (3993 of 4000)
PASS: MLIR :: mlir-runner/utils.mlir (3994 of 4000)
PASS: MLIR :: mlir-tblgen/llvm-intrinsics.td (3995 of 4000)
PASS: MLIR :: Pass/pass-timing.mlir (3996 of 4000)
PASS: MLIR :: mlir-tblgen/cpp-class-comments.td (3997 of 4000)
PASS: MLIR :: Pass/pipeline-parsing.mlir (3998 of 4000)
PASS: MLIR :: Pass/pipeline-options-parsing.mlir (3999 of 4000)
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3121.188670

```

</details>

https://github.com/llvm/llvm-project/pull/181707