[llvm] [AMDGPU] Simplify AMDGPUDisassembler::getInstruction by removing Res. (PR #82775)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Oh, some tests are failing.

Fixed. I was not reporting failure when getInstruction was given less than 4 bytes to disassemble.

https://github.com/llvm/llvm-project/pull/82775