[llvm] [X86] Remove zero extends made redundant by the byte/word fixup (PR #217923)

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

Do cheap iterator checks first?
```suggestion
      if (It != Known.end() && It->second <= ReadBits &&
          getX86SubSuperRegister(SrcSuper, ReadBits) == Src) {
```

https://github.com/llvm/llvm-project/pull/217923