image_sample_*_o takes its texel offset from an address VGPR. The emitter
accepted only an immediate there and failed with "requires a constant texel
offset for image sampling" for any other, including an offset the program
moves into the VGPR under EXEC.
An offset that resolves to a constant at its packed address position stays a
ConstOffset. Any other becomes the Offset image operand, which Vulkan allows
on sampling instructions only with VK_KHR_maintenance8's maintenance8 feature
and the ImageGatherExtended capability. The device enables maintenance8 when
available and reports it through SpirvTarget::nonConstantImageOffsets, which
the cache key, the request serialization (version 6) and the SPIRV-Tools
validator options follow. Without it the sample still fails.
agc_shader_replay --maintenance8 recompiles a request for such a device.
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