Add a Rust NullCheckElimination pass #13

This pass is not Rust-specific, in that all of its transformations are intended to be correct for arbitrary LLVM IR, but it targets idioms found in IR generated by `rustc`, e.g. heavy use of `inbounds` GEPs.

Since the NullCheckElimination pass has a similar intent to the CorrelatedValuePropagation pass, I decided to run it right after the both places that the latter runs.