AMDGCN flavoured SPIR-V has slightly different defaults from what the BE
adopts: it assumes all extensions are enabled, and expects nonsemantic
debug info to be generated. Furthermore, it is necessary to encode in
the resulting SPIR-V binary that what was generated was AMDGCN
flavoured, which we do by setting the Generator Version to `UINT16_MAX`
(which matches what we expect to see at reverse translation). We will
register this generator version at
<https://github.com/KhronosGroup/SPIRV-Headers>. This is a preliminary
patch out of a series of patches that are needed for adopting the BE for
AMDGCN flavoured SPIR-V generation.
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