[HLSL] Implement Texture2D::Load methods and builtin
Implements the Textur2D::Load methods. A new HLSL buildin is added to
implement the method. The HLSL builtin is lowered to the
resource_load_level intrinsic.
We chose to have have a single operand hold both the coordinate and the
level in the builtin, as is done in the Load method itself. This was to
make the external sema source easier. It is easier to split the vector
during codegen than in sema.
Assisted-by: Gemini
