[llvm] [NFC][SPIRV] Add AMDGCN SPIR-V specific defaults to the BE (PR #165815)

[Juan Manuel Martinez Caamaño via llvm-commits]

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jmmartinez wrote:

```cpp
case Triple::SPIRVSubArch_v14:
  SPIRVVersion = VersionTuple(1, 4);
  break;
case Triple::SPIRVSubArch_v15:
  SPIRVVersion = VersionTuple(1, 5);
  break;
case Triple::SPIRVSubArch_v16:
  SPIRVVersion = VersionTuple(1, 6);
  break;
default:
    if (TT.getVendor() == Triple::AMD)
      SPIRVVersion = VersionTuple(1, 6);
    else
      SPIRVVersion = VersionTuple(1, 4);
}
```

This doesn't look right (but it's because the switch just puts the default mixed with the case for 1.4.

If v1.4 was explicitly specified we should still assign Version 1.4. We should move the default away.


https://github.com/llvm/llvm-project/pull/165815