Cleanup dead blocks after noreturn folding

Follow-up passes might not handle blocks with undef instruction operands
well.

rdar://41139395

Author: aschwaighofer

lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp

@@ -20,6 +20,7 @@
 #include "swift/SIL/SILBuilder.h"
 #include "swift/SIL/SILUndef.h"
 #include "swift/SILOptimizer/Utils/Local.h"
+#include "swift/SILOptimizer/Utils/CFG.h"
 #include "swift/SILOptimizer/PassManager/Transforms.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/Statistic.h"
@@ -750,13 +751,16 @@ static void performNoReturnFunctionProcessing(SILFunction &Fn,
                                               SILFunctionTransform *T) {
   DEBUG(llvm::errs() << "*** No return function processing: " << Fn.getName()
                      << "\n");
-
+  bool Changed = false;
   for (auto &BB : Fn) {
     // Remove instructions from the basic block after a call to a noreturn
     // function.
-    simplifyBlocksWithCallsToNoReturn(BB, nullptr);
+    Changed |= simplifyBlocksWithCallsToNoReturn(BB, nullptr);
+  }
+  if (Changed) {
+    removeUnreachableBlocks(Fn);
+    T->invalidateAnalysis(SILAnalysis::InvalidationKind::FunctionBody);
   }
-  T->invalidateAnalysis(SILAnalysis::InvalidationKind::FunctionBody);
 }
 
 static void diagnoseUnreachable(SILFunction &Fn) {

test/SILOptimizer/mandatory_inlining.swift

@@ -175,3 +175,16 @@ public class A {
     }
   }
 }
+
+// This used to crash during mandatory inlining because noreturn folding would
+// create sil instructions with undef in unreachable code.
+func dontCrash() {
+  fatalError() // expected-note {{a call to a never-returning function}}
+  let k = "foo" // expected-warning {{will never be executed}}
+  switch k {
+  case "bar":
+    return
+  default:
+    fatalError("baz \(k)")
+  }
+}