[test] Fix TBI test for SE-0055 (optional pointers).

(TBI = "top byte ignore")

Author: jrose-apple

test/Misc/tbi.swift

@@ -1,20 +1,25 @@
 // RUN: %target-build-swift -target arm64-apple-ios8.0 -target-cpu cyclone \
-// RUN:   -O -S %s -parse-as-library | \
+// RUN:   -O -S %s -parse-as-library -parse-stdlib | \
 // RUN:   FileCheck --check-prefix=TBI %s
 
 // RUN: %target-build-swift -target arm64-apple-ios7.0 -target-cpu cyclone \
-// RUN:     -O -S %s -parse-as-library | \
+// RUN:     -O -S %s -parse-as-library -parse-stdlib | \
 // RUN:   FileCheck --check-prefix=NO_TBI %s
 
 // REQUIRES: CPU=arm64, OS=ios
 
 // Verify that TBI is on by default in Swift on targets that support it. For our
 // purposes this means iOS8.0 or later.
 
+import Swift
+
 func f(i: Int) -> Int8 {
   let j = i & 0xff_ffff_ffff_ffff
 // TBI-NOT: and
 // NO_TBI: and
-  let p = UnsafeMutablePointer<Int8>(bitPattern: j)
+  guard let p = UnsafeMutablePointer<Int8>(bitPattern: j) else {
+    // Make sure we optimize away this branch.
+    Builtin.unreachable()
+  }
   return p[0]
 }