Separate instruction simplification from insertion into DFG

[TomAFrench]

We currently don't have a particularly good way to create exact SSA programs as we automatically perform any available simplifications as we push instructions into the function. This has been flagged as an issue in #6434 and has been something that I've definitely run into a couple of times.

It would then be nice to be able to add a flag to the FunctionBuilder so that we can get it to skip any instruction simplification when constructing a function which would allow us to construct test programs more easily.