Remove old issues and improve labeling

@fitzgen @abrown @cfallin @akirilov-arm @bjorn3 @alexcrichton @alexavanhattun

This issue is being created to write thoughts, acknowledge, track and agree on TODOs for scrubbing obsolete compiler issues and improving labeling. This is an issue that hopefully we can complete by EOY. Yes, I think there is irony in creating an issue to clean up unnecessary issues. The oldest issue # starts at 1022 and the most recent goes to 3562. As @akirilov-arm and @bjorn3 was noted, there are plenty of issues here that aren't cranelift specific but as @cfallin noted, as long as they are compiler related we should feel free to scrub. Action items that we loosely proposed are:

#1 go through and start closing issues that we think are obsolete. We should mention the issue creator and/or a domain expert as we close.
#2 for items we close and items we keep open, we should make sure labels are up-to-date as best we are aware.
#3 we probably don't need to assign a block of issues to any one person. It will make sense for a person to triage items related to areas of code they are most familiar with. if you'd like to be mentioned on certain issue types before they are deleted, maybe you can mention so here in this thread.
#4 Labels may not make sense to everyone and when no one understand what a label means they are likely to not use the label and maybe even ignore issues with those labels. On that note, I think we should have at least 2 items here that someone note they'll take ownership of (1) rename/combine/redescribe the cranelift:E-* labels and (2) combine the cranelift:docs and cranelift:documentation labels.