This Pull Request adds OpImageWrite as defined in section 3.52.10.
(Image Instructions). The tests in
`mlir/test/Target/SPIRV/image-ops.mlir` are also updated (and extended
with the new op), so they now pass validation with `spirv-val` after
serialization into SPIR-V. The test was missing `ImageQuery` capability
and entry points. For entry points dummy `main` functions were added.