[llvm] DAG: Emit an error if trying to legalize read/write register with illegal types (PR #145197)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Why is a new DK_* required? Doesn't DAG already have a way to report failure to legalize?

No, this is the start of a more proper way. Mostly it does report_fatal_error now. The default emitError doesn't include any useful location information. Really we could use several more types. We could use one to error in missing support cases (the usual case), or cases like this where it will really never work. Plus most of the diagnostic types we have assume a function context, we could use one for failures on global references 

https://github.com/llvm/llvm-project/pull/145197