Replace zero-value constants with OpConstantNull

[antiagainst]

For example, the following SPIR-V constant:

[[vec:%\d+]] = OpConstantComposite %v4float %float_0 %float_0 %float_0 %float_0
[[mat:%\d+]] = OpConstantComposite %mat2v4float [[vec]] [[vec]]

Can be turned into

[[mat:%\d+]] = OpConstantNull %mat2v4float

Shorter codegen; and I assume the performance should be at least on par?

[dnovillo]

This is folding plus constant propagation.

[dneto0]

One place this can't be done is when that integer is used to index a struct in an access-chain.
However, this is controversial: KhronosGroup/SPIRV-Headers#50

[s-perron]

There is not great value in doing this. We will not fix this.