ByteCode Updward Exposed mismatch after DeadStore

Git Commit: 861a276c346746f3b87347bb094988563d7ee0a7
Ubuntu 18.04

PoC:

```
const obj = { };

var a = { toString: ()=> {} };

function opt(o, zz) {
function foo () {
String.prototype.replace.call(o, zz, obj);
}
foo ();
}
opt({}, "zzz");
for (let xi = 0; xi < 500; xi++) {
opt({}, "kkkk");
}

opt(a, "llll")
```

**Assertion failed in debug and segfault in release (possible null pointer dereference)**

```
ByteCode Updward Exposed mismatch after DeadStore
Mismatch Instr:
s32.var = Coerce_StrOrRegex s21[LikelyCanBeTaggedValue_String].var! #0029

ByteCode Updward Exposed mismatch after DeadStore
Mismatch Instr:
Bailout: #0029 (BailOutOnImplicitCallsPreOp)
ByteCode Register list present before Backward pass missing in DeadStore pass:
s53 R5
ASSERT.ION var26087: ( /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp, line 8936) ByteCode Updward Exposed Used Mismatch
Failure: (false)
Illegal instruction
```

stack:

```
#0 0x000055555a82bd66 in BackwardPass::VerifyByteCodeUpwardExposed (this=0x154d4dbf4e00, block=0x15554ea562b0, func=0x154d4dbf6d00, trackingByteCodeUpwardExposedUsed=0x15554ea584a0, instr=0x15554ea5ac40, bytecodeOffset=41) at /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp:8936
#1 0x000055555a820c0f in BackwardPass::ProcessBailOutInfo (this=0x154d4dbf4e00, instr=0x15554ea5ac40, bailOutInfo=0x15554ea5ab20) at /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp:2714
#2 0x000055555a82b0bd in BackwardPass::ProcessPendingPreOpBailOutInfo (this=0x154d4dbf4e00, currentInstr=0x15554ea5ac40) at /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp:2612
#3 0x000055555a812e4a in BackwardPass::ProcessBlock (this=0x154d4dbf4e00, block=0x15554ea562b0) at /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp:3886
#4 0x000055555a7f5f25 in BackwardPass::OptBlock (this=0x154d4dbf4e00, block=0x15554ea562b0) at /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp:1711
#5 0x000055555a7f5168 in BackwardPass::Optimize (this=0x154d4dbf4e00) at /tmp/media/code/ChakraCore/lib/Backend/BackwardPass.cpp:430
#6 0x000055555a1a20fb in GlobOpt::BackwardPass (this=0x154d4dbf5aa0, tag=Js::Phase::DeadStorePhase) at /tmp/media/code/ChakraCore/lib/Backend/GlobOpt.cpp:159
#7 0x000055555a1a3087 in GlobOpt::Optimize (this=0x154d4dbf5aa0) at /tmp/media/code/ChakraCore/lib/Backend/GlobOpt.cpp:212
#8 0x000055555a13a586 in Func::TryCodegen (this=0x154d4dbf6d00) at /tmp/media/code/ChakraCore/lib/Backend/Func.cpp:456
#9 0x000055555a138da2 in Func::Codegen (alloc=0x154d4dbf75a0, workItem=0x15554ea41030, threadContextInfo=0x623000000198, scriptContextInfo=0x622000000158, outputData=0x154d4dbf7b90, epInfo=0x15554ec381b0, runtimeInfo=0x0, polymorphicInlineCacheInfo=0x15554ea88420, codeGenAllocators=0x61a0000006d8, codeGenProfiler=0x0, isBackgroundJIT=true) at /tmp/media/code/ChakraCore/lib/Backend/Func.cpp:324
#10 0x0000555559b323c3 in NativeCodeGenerator::CodeGen (this=0x613000000798, pageAllocator=0x615000002678, workItemData=0x612000000540, jitWriteData=..., foreground=false, epInfo=0x15554ec381b0) at /tmp/media/code/ChakraCore/lib/Backend/NativeCodeGenerator.cpp:894
#11 0x0000555559b3658f in NativeCodeGenerator::CodeGen (this=0x613000000798, pageAllocator=0x615000002678, workItem=0x612000000518, foreground=false) at /tmp/media/code/ChakraCore/lib/Backend/NativeCodeGenerator.cpp:1011
#12 0x0000555559b3d6d8 in NativeCodeGenerator::Process (this=0x613000000798, job=0x612000000520, threadData=0x615000002658) at /tmp/media/code/ChakraCore/lib/Backend/NativeCodeGenerator.cpp:1911
#13 0x0000555559c800b4 in JsUtil::BackgroundJobProcessor::Process (this=0x612000000218, job=0x612000000520, threadData=0x615000002658) at /tmp/media/code/ChakraCore/lib/Common/Common/Jobs.cpp:1037
#14 0x0000555559c810ca in JsUtil::BackgroundJobProcessor::Run (this=0x612000000218, threadData=0x615000002658) at /tmp/media/code/ChakraCore/lib/Common/Common/Jobs.cpp:1135
#15 0x0000555559c77f3f in JsUtil::BackgroundJobProcessor::StaticThreadProc (lpParam=0x615000002658) at /tmp/media/code/ChakraCore/lib/Common/Common/Jobs.cpp:1319
#16 0x000055555666c87d in CorUnix::CPalThread::ThreadEntry (pvParam=0x61c000001880) at /tmp/media/code/ChakraCore/pal/src/thread/pal_thread.cpp:1605
#17 0x00001555548bb6db in start_thread (arg=0x154d4dbf9700) at pthread_create.c:463
#18 0x0000155553c22a3f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
```