swift234

Some of the functions do not really need the iterative data flow in DSE. i.e. for function
of which a single post-order would be enough for DSE. In this case, we do not really
need to compute the genset and killset (which is a costly operation).

On stdlib, i see 93% of the functions are "OneIterationFunction".

With this change, i see the compilation time of DSE drops from 2.0% to 1.7% of the entire compilation.
This represents 4.3% of all the time spent in SILOptimizations (39.5%).