[llvm] [CSKY] Use `softPromoteHalfType` (PR #175138)

[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/36169

<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 :: Dialect/XeGPU/ops.mlir (3788 of 3799)
PASS: MLIR-Unit :: Dialect/Utils/./MLIRDialectUtilsTests/13/42 (3789 of 3799)
PASS: MLIR-Unit :: Dialect/Utils/./MLIRDialectUtilsTests/35/42 (3790 of 3799)
PASS: MLIR :: Pass/pipeline-stats-nested.mlir (3791 of 3799)
PASS: MLIR-Unit :: Dialect/Utils/./MLIRDialectUtilsTests/8/42 (3792 of 3799)
PASS: MLIR :: mlir-tblgen/llvm-intrinsics.td (3793 of 3799)
PASS: MLIR-Unit :: Dialect/Utils/./MLIRDialectUtilsTests/39/42 (3794 of 3799)
PASS: MLIR :: mlir-runner/utils.mlir (3795 of 3799)
PASS: MLIR :: mlir-runner/simple.mlir (3796 of 3799)
PASS: MLIR :: Pass/pipeline-options-parsing.mlir (3797 of 3799)
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=1813.961874

```

</details>

https://github.com/llvm/llvm-project/pull/175138