shader: Partially implement texture cube array shadow

This implements texture cube arrays with shadow comparisons but doesn't
fix the asserts related to it.

Fixes out of bounds reads on swizzle constructors and makes them use
bounds checked ::at instead of the unsafe operator[].

src/video_core/shader/decode/texture.cpp

@@ -556,7 +556,6 @@ Node4 ShaderIR::GetTextureCode(Instruction instr, TextureType texture_type,
     const bool is_shadow = depth_compare != nullptr;
     const bool is_bindless = bindless_reg.has_value();
 
-    UNIMPLEMENTED_IF(texture_type == TextureType::TextureCube && is_array && is_shadow);
     ASSERT_MSG(texture_type != TextureType::Texture3D || !is_array || !is_shadow,
                "Illegal texture type");