spirv-fuzz: Flatten conditional branches produces invalid OpSelects pre SPIR-V 1.4

The "flatten conditional branches" transformation turns OpPhi into OpSelect. Before SPIR-V 1.4 this has two problems:

- An OpPhi on something that isn't a scalar or vector (e.g. a matrix won't work) because OpSelect is restricted to scalar and vector types

- An OpPhi on a vector won't work because it requires a vector of booleans as the selector