[LegacyPM] Fix PR37888 by teaching the legacy loop pass manager how to

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335317 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SimpleLoopUnswitch/pr37888.ll

@@ -0,0 +1,38 @@
+; RUN: opt -simple-loop-unswitch -loop-deletion -S < %s | FileCheck %s
+;
+; Check that when we do unswitching where we re-enqueue the loop to be processed
+; again, but manage to delete the loop before ever getting to iterate on it, it
+; doesn't crash the legacy pass manager.
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @pr37888() {
+; CHECK-LABEL: define void @pr37888()
+entry:
+  %tobool = icmp ne i16 undef, 0
+  br label %for.body
+; CHECK:         %[[TOBOOL:.*]] = icmp ne
+; CHECK-NEXT:    br i1 %[[TOBOOL]], label %if.then, label %[[ENTRY_SPLIT:.*]]
+;
+; CHECK:       [[ENTRY_SPLIT]]:
+; CHECK-NEXT:    br label %for.end
+
+for.body:
+  br i1 %tobool, label %if.then, label %if.end
+
+if.then:
+  unreachable
+; CHECK:       if.then:
+; CHECK-NEXT:    unreachable
+
+if.end:
+  br label %for.inc
+
+for.inc:
+  br i1 undef, label %for.body, label %for.end
+
+for.end:
+  ret void
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+}