llvm-reduce: remove a crash in the basic block reducing pass

[regehr]

Summary

this pass should work a lot better now, it was unconditionally removing blocks from predecessor phi nodes, now it is only doing that for in-chunk blocks

the non-default second argument to removePredecessor() is necessary to avoid creating invalid IR on examples like the one in the provided test case

[arsenm]

llvm/test/tools/llvm-reduce/remove-bbs-nocrash.ll

7

Can drop these

llvm/test/tools/llvm-reduce/remove-bbs-nocrash.ll

+; RUN: llvm-reduce -abort-on-invalid-reduction --delta-passes=basic-blocks --test FileCheck --test-arg --check-prefix=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
+
+; CHECK-INTERESTINGNESS: ret
+
+
+
+source_filename = "bad.ll"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @f(ptr nocapture %arg, ptr %arg1) {
+bb:
+  br i1 false, label %bb2, label %bb26
+
+bb2:                                              ; preds = %bb
+  br label %bb4
+
+bb4:                                              ; preds = %bb2
+  br label %bb5
+
+bb5:                                              ; preds = %bb5, %bb4
+  %i = phi i64 [ 0, %bb4 ], [ %i9, %bb5 ]
+  %i6 = getelementptr inbounds i64, ptr %arg, i64 %i
+  %i7 = load i64, ptr %i6, align 8
+  %i8 = getelementptr inbounds i64, ptr %arg1, i64 %i
+  store i64 0, ptr %i8, align 8
+  %i9 = add nuw nsw i64 %i, 1
+  %i10 = icmp eq i64 %i9, 0
+  br i1 %i10, label %bb26, label %bb5
+
+bb26:                                             ; preds = %bb5, %bb
+  ret void
+}

llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp

for (auto &BB : F) {
       if (!okToRemove(BB, F, BBsToDelete) || O.shouldKeep())
         BBsToKeep.insert(&BB);
       else
         BBsToDelete.insert(&BB);
     }
   }
 
   // Remove out-of-chunk BB from successor phi nodes
-  for (auto &F : Program) {
-    for (auto &BB : F) {
-      for (auto *Succ : successors(&BB))
-        Succ->removePredecessor(&BB);
-    }
+  for (auto &BB : BBsToDelete) {
+    for (auto *Succ : successors(BB))
+      Succ->removePredecessor(BB, /*KeepOneInputPHIs=*/true);
   }
 
   // Replace terminators that reference out-of-chunk BBs
   for (auto &F : Program)
     for (auto &BB : F) {
       if (auto *SwInst = dyn_cast<SwitchInst>(BB.getTerminator()))
         removeUninterestingBBsFromSwitch(*SwInst, BBsToKeep);
       else