This also adds the first set of tests for operand bundles.
The optimizer has not been audited to ensure that it does the right
thing with operand bundles.
This change adds a stub to LangRef, but documentation is still a TODO.
Given that this is a large change set, I figured it would be wise to get
this out for review and get some feedback earlier rather than later.
I'll update this patch with more comprehensive documentation soon.
Depends on D12456.