* Disable operators that rely on references

After lots of discussion, we think it is best to disable operators that
should rely on reference parameters or return types for HLSL 2021. The
main driver of this is that we believe that references will get added to
HLSL in the near future, and we'd like to make it easier to maintain
compatability between HLSL 2021 and future versions of HLSL where
references are supported.

* Removing assignment operator test case

(cherry picked from commit ade5e47a94b7e46bbe75288e835d20ae9771329e)