SILOptimizer: fix a stack overflow in DCE

For very large control flow graphs the markControllingTerminatorsLive can stack overflow.
Fix this by doing the work iteratively instead of recursively.

rdar://106198943

Author: eeckstein

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -215,7 +215,6 @@ void DCE::markInstructionLive(SILInstruction *Inst) {
 
   LLVM_DEBUG(llvm::dbgs() << "Marking as live: " << *Inst);
 
-  markControllingTerminatorsLive(Inst->getParent());
   Worklist.push_back(Inst);
 }
 
@@ -450,6 +449,8 @@ void DCE::propagateLiveBlockArgument(SILArgument *Arg) {
 // Given an instruction which is considered live, propagate that liveness
 // back to the instructions that produce values it consumes.
 void DCE::propagateLiveness(SILInstruction *I) {
+  markControllingTerminatorsLive(I->getParent());
+
   if (!isa<TermInst>(I)) {
     for (auto &O : I->getAllOperands())
       markValueLive(O.get());

validation-test/SILOptimizer/large_switch.gyb

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: %gyb %s > %t/main.swift
+// RUN: %target-swift-frontend -O -parse-as-library -sil-verify-none -emit-sil %t/main.swift -o /dev/null
+
+// REQUIRES: swift_stdlib_no_asserts,optimized_stdlib
+// REQUIRES: long_test
+
+// Check that the compiler does not crash in DCE due to a stack overflow.
+// rdar://106198943
+
+public func test(_ a: Int) -> Int {
+  switch (a) {
+% for i in range(32000):
+    case ${i}: return ${i % 7}
+% end
+    default: return -1
+  }
+}
+