[llvm] Instruction: avoid crash in getStableDebugLoc when `this` isn't a DbgInfoIntrinsic (PR #66266)

[Fangrui Song via llvm-commits]

================
@@ -886,8 +886,11 @@ Instruction::getPrevNonDebugInstruction(bool SkipPseudoOp) const {
 }
 
 const DebugLoc &Instruction::getStableDebugLoc() const {
-  if (isa<DbgInfoIntrinsic>(this))
-    return getNextNonDebugInstruction()->getDebugLoc();
+  if (isa<DbgInfoIntrinsic>(this)) {
+    const Instruction* next = getNextNonDebugInstruction();
----------------
MaskRay wrote:

LLVM coding standard prefers `Instruction *next`. With C++17, prefer:

```
if (const Instruction *Next = getNextNonDebugInstruction())
  return Next->getDebugLoc();
```

https://github.com/llvm/llvm-project/pull/66266