microsoft/compiler: Shadow tex instructions always use shadow samplers
Since in SPIR-V the "shadowness" of a sampler is determined by image ops, all samplers start as non-shadow. In DXIL, shadow image ops must use shadow samplers. Therefore, we need to fix all sampler to match their usage. As for situations where a sampler could be used as both shadow and non-shadow, HLSL, GLSL, and WGSL have separate types for shadow samplers and opencl currently does not support shadow operations.