[PATCH] ADCE: Remove debug info intrinsics in dead scopes

[Duncan P. N. Exon Smith via llvm-commits]

During ADCE, track which debug info scopes still have live references
from the code, and delete debug info intrinsics for the dead ones.

These intrinsics describe the locations of variables (in registers or
stack slots).  If there's no code left corresponding to a variable's
scope, then there's no way to reference the variable in the debugger and
it doesn't matter what its value is.

I add a DEBUG printout when the described location in an SSA register,
in case it helps some trying to track down why locations get lost.
However, we still delete these; the root problem is that the scope
itself isn't attached to any real code.

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