Preserving the case order is not strictly necessary to preserve
semantics (for example, operations like SwitchInst::removeCase will
happily swap cases around). However, I'm planning to introduce an
optional verification step for SandboxIR that will use StructuralHash to
compare IR after a revert to the original IR to help catch tracker bugs,
and the order difference triggers a difference there.
`insertBefore` can be called on a detached instruction, and we can't
check that the underlying instructions are ordered because instructions
without BB parents have no order.
This problem showed up as a different assertion failure in
`llvm::Instruction::comesBefore` in one of the unit tests when
`EXPENSIVE_CHECKS` are enabled.
This patch implements the InsertPosition class that is used to specify
where an instruction should be placed.
It also switches a couple of create() functions from the old API to the
new one that uses InsertPosition.
