HLSL: use LOD form of ImageQuerySize when needed.

The non-LOD form of image size query is prohibited in certain cases:
see the OpImageQuerySize and OpImageQuerySizeLod sections of the SPIR-V
spec for details.  Sometimes we were generating the non-LOD form when
we should have been using the LOD form.  Sometimes the LOD form is required
even if the underlying HLSL query did not supply a MIP level itself,
in which case level 0 is now queried.