SimplifyBranch: don't leave an empty block after merging two basic blocks

SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyBranch.swift

@@ -70,8 +70,10 @@ private extension BranchInst {
       }
       parentBB.moveAllInstructions(toBeginOf: targetBB, context)
       parentBB.moveAllArguments(to: targetBB, context)
+      context.erase(block: parentBB)
     } else {
       targetBB.moveAllInstructions(toEndOf: parentBB, context)
+      context.erase(block: targetBB)
     }
   }
 }

SwiftCompilerSources/Sources/Optimizer/PassManager/Context.swift

@@ -71,6 +71,10 @@ extension MutatingContext {
     erase(instruction: inst)
   }
 
+  func erase(block: BasicBlock) {
+    _bridged.eraseBlock(block.bridged)
+  }
+
   func tryOptimizeApplyOfPartialApply(closure: PartialApplyInst) -> Bool {
     if _bridged.tryOptimizeApplyOfPartialApply(closure.bridged) {
       notifyInstructionsChanged()
@@ -227,10 +231,6 @@ struct FunctionPassContext : MutatingContext {
     }
   }
 
-  func erase(block: BasicBlock) {
-    _bridged.eraseBlock(block.bridged)
-  }
-
   func modifyEffects(in function: Function, _ body: (inout FunctionEffects) -> ()) {
     notifyEffectsChanged()
     function._modifyEffects(body)

test/SILOptimizer/simplify_branch_crash.sil

@@ -0,0 +1,28 @@
+// RUN: %target-sil-opt -enable-sil-verify-all %s -sil-opt-pass-count=1.1 -simplification -simplify-instruction=br | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+
+import Swift
+import Builtin
+
+// Check that branch simplification doesn't leave empty blocks.
+// -sil-opt-pass-count=1.1  prevents dead block elimination which would hide the problem.
+
+
+
+// CHECK-LABEL: sil @dont_leave_empty_blocks
+// CHECK:       bb0(%0 : $Builtin.Int64):
+// CHECK-NEXT:    br bb1
+// CHECK:       bb1:
+// CHECK-NEXT:    return %0
+// CHECK:       } // end sil function 'dont_leave_empty_blocks'
+sil @dont_leave_empty_blocks : $@convention(thin) (Builtin.Int64) -> Builtin.Int64 {
+bb0(%0 : $Builtin.Int64):
+  br bb1
+
+bb1:
+  br bb2
+
+bb2:
+  return %0 : $Builtin.Int64
+}