[cast-optimizer] Use a stricter pre-condition

Fixes a compiler crasher reported in rdar://21635259.

Swift SVN r30517

Author: swiftix

lib/SILPasses/Utils/Local.cpp

@@ -1596,7 +1596,10 @@ optimizeCheckedCastAddrBranchInst(CheckedCastAddrBranchInst *Inst) {
         MI = dyn_cast<MetatypeInst>(Src);
 
       if (MI) {
-        if (SuccessBB->getSinglePredecessor()) {
+        if (SuccessBB->getSinglePredecessor()
+            && canUseScalarCheckedCastInstructions(Inst->getModule(),
+                MI->getType().getSwiftRValueType(),
+                Dest.getType().getObjectType().getSwiftRValueType())) {
           SILBuilderWithScope<1> B(Inst);
           auto NewI = B.createCheckedCastBranch(
               Loc, false /*isExact*/, SILValue(MI, 0),

test/SILPasses/cast_folding.swift

@@ -751,6 +751,29 @@ public func callForGenericCC(c: CC) {
   callFoo(c)
 }
 
+// Check that this conversion does not crash a compiler.
+@inline(never)
+public func test34() {
+  if let _ = A.self as? P {
+    print("A: dice")
+  } else {
+    print("A: no dice")
+  }
+}
+
+
+// Check that this conversion does not crash a compiler.
+@inline(never)
+public func test35() {
+  if let _ = X.self as? P {
+    print("X: dice")
+  } else {
+    print("X: no dice")
+  }
+}
+
+
+
 print("test0=\(test0())")
 
 print("test1=\(test1())")