Add support for inbounds, nsw, nuw, exact flags

[Benjins]

Is your feature request related to a problem? Please describe.
The instruction-specific flags (e.g. inbounds, nsw, nuw, exact) do not seem to be exposed (I could not find any docs on them, and I don't see the relevant llvm-sys API calls in the code). Specifically, add/sub/mul instructions can have nuw/nsw flags, udiv/sdiv/urem/srem instructions can have the exact flag, and getelementptr instructions can have the inbounds flag. These should all now be exposed via llvm-sys, though some only in more recent LLVM versions

Describe the solution you'd like
This information should be exposed somehow. I see that the fast-math flags, disjoint, volatile, etc. are exposed on the InstructionValue struct. That seems like a logical place to expose these as well, unless there's another place that is considered more appropriate or there are concerns about InstructionValue taking on too much surface

Additional context
LangRef - Binary arithmetic ops
LangRef - GEP

There are likely other flags coming down the pipeline: the samesign flag for icmp instructions, for example

If it's as simple as adding getter/setter methods to InstructionValue, I can take a crack at a PR. Figured it was worth opening an issue to see if there were other thoughts