clspv::ComputeStructuredOrder crashes on infinite loops

The following IR crashes clspv in clspv::ComputeStructuredOrder. It was produced while reducing another issue using bugpoint.

```
target datalayout = "e-p:32:32-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024"
target triple = "spir-unknown-unknown"

define dso_local spir_kernel void @kernel() {
entry:
br label %for.cond

for.cond: ; preds = %for.cond, %entry
br label %for.cond
}
```

GDB shows:
```
Reading symbols from clspv...
(gdb) r
Starting program: build_shared_debug/bin/clspv -x ir reduced.ll
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
clspv: clspv/lib/ComputeStructuredOrder.cpp:58: void clspv::ComputeStructuredOrder(llvm::BasicBlock*, llvm::DominatorTree*, const llvm::LoopInfo&, std::deque*, llvm::DenseSet*):
Assertion `continue_block && merge_block && "Bad loop"' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff7063fb7 in raise () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0 0x00007ffff7063fb7 in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1 0x00007ffff7065921 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2 0x00007ffff705548a in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#3 0x00007ffff7055502 in __assert_fail () from /lib/x86_64-linux-gnu/libc.so.6
#4 0x00007ffff77655a7 in clspv::ComputeStructuredOrder (block=0x55555579c860, DT=0x5555557a7450, LI=..., order=0x7fffffffab00,
visited=0x7fffffffaa70) at clspv/lib/ComputeStructuredOrder.cpp:58
#5 0x00007ffff776573b in clspv::ComputeStructuredOrder (block=0x55555579c6b0, DT=0x5555557a7450, LI=..., order=0x7fffffffab00,
visited=0x7fffffffaa70) at clspv/lib/ComputeStructuredOrder.cpp:87
#6 0x00007ffff77c5563 in (anonymous namespace)::SPIRVProducerPass::PopulateStructuredCFGMaps (this=0x5555557ee9e0)
at clspv/lib/SPIRVProducerPass.cpp:5667
#7 0x00007ffff77b31aa in (anonymous namespace)::SPIRVProducerPass::runOnModule (this=0x5555557ee9e0, M=...)
at clspv/lib/SPIRVProducerPass.cpp:737
#8 0x00007ffff2bf9059 in (anonymous namespace)::MPPassManager::runOnModule (this=0x55555579dff0, M=...)
at clspv/third_party/llvm/llvm/lib/IR/LegacyPassManager.cpp:1550
#9 0x00007ffff2bf4241 in llvm::legacy::PassManagerImpl::run (this=0x5555557a57b0, M=...)
at clspv/third_party/llvm/llvm/lib/IR/LegacyPassManager.cpp:541
#10 0x00007ffff2bf98e3 in llvm::legacy::PassManager::run (this=0x7fffffffafe0, M=...)
at clspv/third_party/llvm/llvm/lib/IR/LegacyPassManager.cpp:1677
#11 0x00007ffff785b6e7 in clspv::Compile (argc=4, argv=0x7fffffffdd68) at clspv/lib/Compiler.cpp:941
#12 0x00005555555547ca in main (argc=4, argv=0x7fffffffdd68) at clspv/tools/driver/main.cpp:18
```

Reading https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#StructuredControlFlow I'm not sure this construct can be supported. However, clspv should not crash but instead report an error if that's the case.

(This issue and #702 may be duplicate, but I was not able to get 100% confidence that's the case.)