fix a spir-val error in the cooperative_matrix_constant_null test (#3568)
Fixes a validation error reported by recent versions of spirv-val:
```
error: line 13: OpTypeCooperativeMatrixKHR Scope is limited to Workgroup and Subgroup
%8 = OpTypeCooperativeMatrixKHR %uint %3 %uint_12 %uint_12 %3
```
Basically, the cooperative matrix scope needs to be 2 (Workgroup) or 3
(Subgroup) instead of the current 0 (CrossDevice).
Since 3 (Subgroup) is the most common usage, for now at least, I've gone
with it.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/30258ee35493b53