Before this commit `StoreInst` weren't supported because they weren't
expected to reach the late stages of the decompilation pipeline where
the `hasSideEffects` helper function is used.
They were expected to be replaced by custom opcodes marked with
`FunctionTags::Assign`.
With the upcoming switch to the new decompilation pipeline, this helper
needs to be able to handle `StoreInst`s, because the custom opcode
functions will be dropped in favor of regular LLVM memory accesses.