clear out deleted loops from the current queue beyond just the current
loop.
This is important because SimpleLoopUnswitch will now enqueue the same
loop to be re-processed. When it does this with the legacy PM, we don't
have a way of canceling the rest of the pipeline and so we can end up
deleting the loop before we reprocess it. =/
This change also makes it easy to support deleting other loops in the
queue to process, although I don't have any use cases for that.
Differential Revision: https://reviews.llvm.org/D48470
llvm-svn: 335317
