[SimpleLoopUnswitch] remove a chain of dead blocks at once

Recent change to deleteDeadBlocksFromLoop was not enough to
fix all the problems related to dead blocks after nontrivial
unswitching of switches.

We need to delete all the dead blocks that were created during
unswitching, otherwise we will keep having problems with phi's
or dead blocks.

This change removes all the dead blocks that are reachable from the loop,
not trying to track whether these blocks are newly created by unswitching
or not. While not completely correct, we are unlikely to get loose but
reachable dead blocks that do not belong to our loop nest.

It does fix all the failures currently known, in particular PR38778.

Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D51519

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341398 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SimpleLoopUnswitch/delete-dead-blocks.ll

@@ -43,3 +43,59 @@ get_out:
 get_out2:
   unreachable
 }
+
+;
+; This comes from PR38778
+; CHECK-LABEL: @Test2
+define void @Test2(i32) {
+header:
+  br label %loop
+loop:
+  switch i32 %0, label %continue [
+    i32 -2147483648, label %check
+    i32 98, label %guarded1
+    i32 99, label %guarded2
+  ]
+; CHECK-NOT: {{^}}guarded1:
+guarded1:
+  br i1 undef, label %continue, label %leave
+guarded2:
+  br label %continue
+check:
+  %val = add i32 0, 1
+  br i1 undef, label %continue, label %leave
+continue:
+  br label %loop
+leave:
+  %local = phi i32 [ 0, %guarded1 ], [ %val, %check ]
+  ret void
+}
+
+;
+; Yet another test from PR38778
+;
+; CHECK-LABEL: @Test3
+define void @Test3(i32) {
+header:
+  br label %outer
+outer:
+  %bad_input.i = icmp eq i32 %0, -2147483648
+  br label %inner
+inner:
+  br i1 %bad_input.i, label %overflow, label %switchme
+overflow:
+  br label %continue
+switchme:
+  switch i32 %0, label %continue [
+    i32 88, label %go_out
+    i32 99, label %case2
+  ]
+; CHECK-NOT: {{^}}case2:
+case2:
+  br label %continue
+continue:
+  %local_11_92 = phi i32 [ 0, %switchme ], [ 18, %case2 ], [ 0, %overflow ]
+  br i1 undef, label %outer, label %inner
+go_out:
+  unreachable
+}