spirv-opt: Fix OpCompositeInsert with Null Constant

[spencer-lunarg]

closes #5005

Now when folding OpCompositeInsert with a null composite it correctly handles it.

The main issue is class NullConstant : public Constant {} is not a CompositeConstant because sometimes OpConstantNull can be either. For cases where there is a OpConstantNull %v3float it will turn it into a OpConstantComposite %v3float %null %null %null before continuing with the logic as normal

cc @sugoi1 I hope this fixes everything now

[sugoi1]

Thanks @spencer-lunarg. I'll try SPIRV-Tools' new version as soon as this lands.

[sugoi1]

Note: @spencer-lunarg, you could also ask alan-baker or dneto0 to review, in case one of them is available earlier than dnovillo.

[dnovillo]

Just a documentation nit. Thanks for the fix.