C2's conditional move optimization does not handle top Phi

This issue has been reported by John Jiang (johnsjiang@tencent.com).

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/oracle/jdk/open/src/hotspot/share/opto/movenode.cpp:175), pid=777849, tid=777864
# Error: ShouldNotReachHere()

Current CompileTask:
C2: 300 26 4 compiler.loopopts.TestCMoveWithDeadPhi::test (58 bytes)

Stack: [0x00007f29cac36000,0x00007f29cad37000], sp=0x00007f29cad31180, free space=1004k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x14d6076] CMoveNode::make(Node*, Node*, Node*, Node*, Type const*)+0x66
V [libjvm.so+0x132202d] PhaseIdealLoop::conditional_move(Node*) [clone .part.0]+0xc8d
V [libjvm.so+0x1325557] PhaseIdealLoop::split_if_with_blocks_pre(Node*)+0x117
V [libjvm.so+0x13273f7] PhaseIdealLoop::split_if_with_blocks(VectorSet&, Node_Stack&)+0x137
V [libjvm.so+0x131b91d] PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0x121d
V [libjvm.so+0xa401fb] PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x32b
V [libjvm.so+0xa3c9d5] Compile::Optimize()+0x14f5
V [libjvm.so+0xa3f1b6] Compile::Compile(ciEnv*, ciMethod*, int, bool, bool, bool, bool, DirectiveSet*)+0x2276
V [libjvm.so+0x86085a] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x1ea
V [libjvm.so+0xa4f349] CompileBroker::invoke_compiler_on_method(CompileTask*)+0xfb9
V [libjvm.so+0xa500b8] CompileBroker::compiler_thread_loop()+0x5a8
V [libjvm.so+0x18b9e11] JavaThread::thread_main_inner()+0x271
V [libjvm.so+0x18c1850] Thread::call_run()+0x100
V [libjvm.so+0x1597bbe] thread_native_entry(Thread*)+0x10e