spirv: Properly handle SpvOpAtomicCompareExchangeWeak

spirv: Properly handle SpvOpAtomicCompareExchangeWeak

The code was handling the Weak variant in some cases, but missing
others, e.g. the get_deref_nir_atomic_op.  Add all the missing cases
with the same behavior of the non-Weak SpvOpAtomicCompareExchange.

Note that the Weak variant is basically an alias, as SPIR-V 1.3,
Revision 7 says

    "OpAtomicCompareExchangeWeak

    Deprecated (use OpAtomicCompareExchange).

    Has the same semantics as OpAtomicCompareExchange."

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>