[X86] Avoid returning the same shuffle operation for broadcast (#70592)

This is to fix a crash since aab8b2eb, which generates a new pattern
```
      t35: v8i32 = xor t11, t14
    t36: v8i32 = vector_shuffle<0,1,0,1,0,1,0,1> t35, undef:v8i32
```

The pattern exposed a bug introduced since f885c080, which breaks
element widen but doesn't handle the broadcast case.

The patch just solved the crash issue. I observed performance regression
cased by above patches in the test, which may need further
investigation.