Improve check in NodeHoisting transformation

[phate]

The current check in the NodeHoisting transformation enables a node to be hoisted if it does not have any state edges. I believe that this is a bit risky as the compiler evolves as the semantics of nodes etc. changes. We already had bugs because of this as exemplified by #1421

I would rather like to switch to a check that explicitly enables only specific operation types to be hoisted. This should be more conservative and hopefully leading to fewer bugs. It might lead to missed hoisting opportunities, but I rather take those than actual bugs.

What do you think @haved ?

[haved]

@phate
We could definitely have some sort of operation flag system, and I think it should be more general than a list that is hardcoded into the NodeHoisting transformation.

I'm for example thinking about how the AllocaOperation has a special implementation of operator == to prevent CNE from eliminating allocas. There could instead be a centralized set of flags for operations, that the different passes can query before making changes, either for correctness or just for performance.

Some flags that could make sense are:

• speculatable -> you can safely hoist it out of conditional code (gamma)
• unique -> you can not deduplicate it. I guess this only applies to allocas for now (?)

In most cases the state edges already kind of tell us whether or not something is pure, but it is not really purity we are after here. For example, a load is not pure, but we probably want to be able to hoist it out of conditional code, if there is no IOBarrier preventing us from doing so.