[llvm] [AMDGPU] Fix a decoder crash in disassembler (PR #214969)

[Shilei Tian via llvm-commits]

shiltian wrote:

* #214969 👈
* `main`

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This stack of pull requests is managed by <a href="https://github.com/shiltian/sgh"><b>sgh</b></a>.
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https://github.com/llvm/llvm-project/pull/214969