Summary:
These allow you to get and set the operator of a dag node, without
affecting its list of arguments.
`!getop` is slightly fiddly because in many contexts you need its
return value to have a static type more specific than 'any record'. It
works to say `!cast<BaseClass>(!getop(...))`, but it's cumbersome, so
I made `!getop` take an optional type suffix itself, so that can be
written as the shorter `!getop<BaseClass>(...)`.
Reviewers: hfinkel, nhaehnle
Reviewed By: nhaehnle
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D71191
