[llvm] X86: Do not return invalid cost for fp16 conversion (PR #114128)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-offload-libc-amdgpu-runtime` running on `omp-vega20-1` while building `llvm` at step 11 "Add check check-libc-amdgcn-amd-amdhsa".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/73/builds/7729

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

```
Step 11 (Add check check-libc-amdgcn-amd-amdhsa) failure: 1200 seconds without output running [b'ninja', b'-j 32', b'check-libc-amdgcn-amd-amdhsa'], attempting to kill
...
[2561/2681] Linking CXX executable libc/test/src/string/libc.test.src.string.strcpy_test.__hermetic__.__build__
[2562/2681] Linking CXX executable libc/test/src/locale/libc.test.src.locale.localeconv_test.__hermetic__.__build__
[2563/2681] Linking CXX executable libc/test/src/inttypes/libc.test.src.inttypes.strtoumax_test.__hermetic__.__build__
[2564/2681] Linking CXX executable libc/test/src/stdlib/libc.test.src.stdlib.strtof_test.__hermetic__.__build__
[2565/2681] Linking CXX executable libc/test/src/string/libc.test.src.string.strncat_test.__hermetic__.__build__
[2566/2681] Linking CXX executable libc/test/src/string/libc.test.src.string.memmove_test.__hermetic__.__build__
[2567/2681] Linking CXX executable libc/test/src/inttypes/libc.test.src.inttypes.strtoimax_test.__hermetic__.__build__
[2568/2681] Linking CXX executable libc/test/src/time/libc.test.src.time.clock_gettime_test.__hermetic__.__build__
[2569/2681] Linking CXX executable libc/test/src/stdio/libc.test.src.stdio.asprintf_test.__hermetic__.__build__
[2570/2681] Linking CXX executable libc/test/src/stdio/libc.test.src.stdio.vasprintf_test.__hermetic__.__build__
command timed out: 1200 seconds without output running [b'ninja', b'-j 32', b'check-libc-amdgcn-amd-amdhsa'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1470.255292

```

</details>

https://github.com/llvm/llvm-project/pull/114128