[TableGen] Add bang-operators !getop and !setop.

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