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

[Nathan Gauër via llvm-commits]

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@@ -17,8 +18,10 @@ using namespace llvm;
 void SPIRVObjectWriter::writeHeader(const MCAssembler &Asm) {
   constexpr uint32_t MagicNumber = 0x07230203;
   constexpr uint32_t GeneratorID = 43;
-  constexpr uint32_t GeneratorMagicNumber =
-      (GeneratorID << 16) | (LLVM_VERSION_MAJOR);
+  const uint32_t GeneratorMagicNumber =
+      Asm.getContext().getTargetTriple().getVendor() == Triple::AMD
+          ? UINT16_MAX
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Keenuts wrote:

nit: Isn't AMD's generator magic number `10`? 

https://github.com/KhronosGroup/SPIRV-Headers/blob/f2e4bd213104fe323a01e935df56557328d37ac8/include/spirv/spir-v.xml#L34

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