<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/55224>55224</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [X86] Uninitialized variable warnings in cetintrin.h
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:X86,
            clang:headers
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          RKSimon
      </td>
    </tr>
</table>

<pre>
    https://godbolt.org/z/bqxb7fdvr

We see these with "-Wall -Wsystem-headers" builds:

```
/opt/compiler-explorer/clang-assertions-trunk-20220502/lib/clang/15.0.0/include/cetintrin.h:47:32: warning: variable 't' is uninitialized when used here [-Wuninitialized]
  return __builtin_ia32_rdsspd(t);
                               ^
/opt/compiler-explorer/clang-assertions-trunk-20220502/lib/clang/15.0.0/include/cetintrin.h:46:17: note: initialize the variable 't' to silence this warning
  unsigned int t;
                ^
/opt/compiler-explorer/clang-assertions-trunk-20220502/lib/clang/15.0.0/include/cetintrin.h:57:32: warning: variable 't' is uninitialized when used here [-Wuninitialized]
  return __builtin_ia32_rdsspq(t);
                               ^
/opt/compiler-explorer/clang-assertions-trunk-20220502/lib/clang/15.0.0/include/cetintrin.h:56:23: note: initialize the variable 't' to silence this warning
  unsigned long long t;
                      ^
```
Noticed while triaging #35297

CC @phoebewang 

</pre>
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