zink/nir-to-spirv: Make sure sampleid for InterpolateAtSample is int

This fixes the validation error VUID-VkShaderModuleCreateInfo-pCode-08737 triggered by piglit:

• spec@arb_gpu_shader5@execution@built-in-functions@fs-interpolateatsample-block-array:

  GLSL.std.450 InterpolateAtSample: expected Sample to be 32-bit integer
    %47 = OpExtInst %float %1 InterpolateAtSample %45 %float_0

Fixes: 9f6be8ef zink: store and use alu types for ntv defs