While working on D79571 I noticed I could change parts of validThroughout
without causing any test failures (*). I'll try to come up with some more tests
if I continue working in this area.
(*) llvm/test/DebugInfo/MIR/AArch64/implicit-def-dead-scope.mir may
fail under similar conditions as one of the checks here, but I don't think it
will catch them both.