tu: weird fail in packing tests
as of !28580 (merged), these tests fail on all devices:
dEQP-GLES3.functional.shaders.builtin_functions.pack_unpack.packunorm2x16_lowp_fragment,Fail
dEQP-GLES3.functional.shaders.builtin_functions.pack_unpack.packunorm2x16_mediump_fragment,Fail
dEQP-GLES3.functional.shaders.builtin_functions.pack_unpack.packunorm2x16_highp_fragment,Fail
As near as I can tell, the shaders are identical save for VAR0 being scalarized after this MR.