[AutoDiff] Re-enable control_flow.swift test.

This test was disabled in SR-12741 due to iphonesimulator-i386 failures.
Enabling the test on other platforms is important to prevent regressions.

Author: dan-zheng

test/AutoDiff/validation-test/control_flow.swift

@@ -1,7 +1,9 @@
 // RUN: %target-run-simple-swift
 // REQUIRES: executable_test
 
-// REQUIRES: SR12741
+// FIXME(SR-12741): Enable test for all platforms after debugging
+// iphonesimulator-i386-specific failures.
+// REQUIRES: CPU=x86_64
 
 import _Differentiation
 import StdlibUnittest