[llvm] [llvm-isel-fuzzer] Fix crash in `IRMutator::getEffectiveTerminator` (PR #180853)

[Demetrius Kanios via llvm-commits]

https://github.com/QuantumSegfault created https://github.com/llvm/llvm-project/pull/180853

`IRMutator::getEffectiveTerminator` would assume that when an UnreachableInst is the terminator of a block, that there is always a previous instruction. This is not the case (you can have a unreachable isolated in a block).

This swaps `dyn_cast` with `dyn_cast_if_present` accordingly.

>From a84b2ca2eb02d5efeeb405a6a87e5fc24d94afc1 Mon Sep 17 00:00:00 2001
From: Demetrius Kanios <demetrius at kanios.net>
Date: Tue, 10 Feb 2026 14:08:51 -0800
Subject: [PATCH] Fix crash in `IRMutator::getEffectiveTerminator`

---
 llvm/lib/FuzzMutate/IRMutator.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/FuzzMutate/IRMutator.cpp b/llvm/lib/FuzzMutate/IRMutator.cpp
index d1abf78222c8c..75ee2611065c2 100644
--- a/llvm/lib/FuzzMutate/IRMutator.cpp
+++ b/llvm/lib/FuzzMutate/IRMutator.cpp
@@ -26,6 +26,7 @@
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/PassInstrumentation.h"
 #include "llvm/IR/Verifier.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Transforms/Scalar/DCE.h"
@@ -124,7 +125,7 @@ static inline Instruction *getEffectiveTerminator(BasicBlock &BB) {
     // Certain intrinsics, such as @llvm.amdgcn.cs.chain, must be immediately
     // followed by an unreachable instruction..
     if (UnreachableInst *UI = dyn_cast<UnreachableInst>(BB.getTerminator())) {
-      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(UI->getPrevNode())) {
+      if (IntrinsicInst *II = dyn_cast_if_present<IntrinsicInst>(UI->getPrevNode())) {
         return II;
       }
     }