[llvm] [SandboxIR] Implement UnreachableInst (PR #101856)

[Julius Alexandre via llvm-commits]

medievalghoul wrote:

@vporpo Apologies for the inconvenience and wait, didn't mean to aruptly commit those recent changes. I'm just now seeing your [comment](https://github.com/llvm/llvm-project/pull/101856#discussion_r1703459647) about wanting to see the backtrace. 

<details>
<summary> Backtrace </summary>

```
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x10)
  * frame #0: 0x0000000100099950 SandboxIRTests`llvm::sandboxir::Instruction::getPrevNode() const + 20
    frame #1: 0x000000010009bf54 SandboxIRTests`llvm::sandboxir::UnreachableInst::create(llvm::sandboxir::Instruction*, llvm::sandboxir::Context&) + 36
    frame #2: 0x0000000100033c40 SandboxIRTests`SandboxIRTest_UnreachableInst_Test::TestBody() + 772
    frame #3: 0x0000000100206114 SandboxIRTests`testing::Test::Run() + 528
    frame #4: 0x0000000100206fa0 SandboxIRTests`testing::TestInfo::Run() + 608
    frame #5: 0x0000000100207be8 SandboxIRTests`testing::TestSuite::Run() + 988
    frame #6: 0x0000000100216098 SandboxIRTests`testing::internal::UnitTestImpl::RunAllTests() + 2220
    frame #7: 0x0000000100215774 SandboxIRTests`testing::UnitTest::Run() + 124
    frame #8: 0x00000001001f8170 SandboxIRTests`main + 148
    frame #9: 0x000000018adea0e0 dyld`start + 2360
```
</details>

https://github.com/llvm/llvm-project/pull/101856