Add `PostInlineHelpersVerifyPass`, a `ModulePass` derived class that
aborts on any `getelementptr` instruction found in any Isolated
function. `inline-helpers` is the only pass in the post-isolate chain
that could import a GEP into an Isolated function by inlining a helper
body that contains one, so the verifier is invoked at the end of
`InlineHelpersPass::runOnModule`. In this way, we early catch this
invariant on the IR.
Andrea Gussoni