[CHR] Fix crash when marking merged condition unknown (#173902)

CHR builds the merged hot-path predicate with
IRBuilder::CreateLogicalAnd. That helper is implemented as a select and
can constant-fold to a non- Instruction (e.g. i1 true). The pass then
attempted to mark the merged condition as having explicitly unknown
branch weights when profile data is present, but it unconditionally did
cast<Instruction>(MergedCondition), which can crash in release builds.

Guard the metadata update with dyn_cast<Instruction> and pass the
containing Function explicitly to avoid calling Instruction::getFunction
when the value is not attached yet.

Add a regression test that exercises the constant-folding case.

Crashing stack:

```
  2.      Running pass "chr" on function "repro_crash"
  #0 0x0000000003be00a4 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (bin/opt+0x3be00a4)
  #1 0x0000000003bdd9e8 llvm::sys::RunSignalHandlers() (bin/opt+0x3bdd9e8)
  #2 0x0000000003be1300 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
  intel#3 0x0000ffffa8e1d840 (linux-vdso.so.1+0x840)
  intel#4 0x0000000003c815e0 llvm::Instruction::getFunction() const (bin/opt+0x3c815e0)
  intel#5 0x0000000003dcd35c llvm::setExplicitlyUnknownBranchWeightsIfProfiled(llvm::Instruction&, llvm::StringRef, llvm::Function const*) (bin/opt+0x3dcd35c)
  intel#6 0x0000000004fb3670 (anonymous namespace)::CHR::addToMergedCondition(bool, llvm::Value*, llvm::Instruction*, (anonymous namespace)::CHRScope*, llvm::IRBuilder<llvm::ConstantFolder, llvm::IRBuilderDefaultInserter>&, llvm::Value*&) ControlHeightReduction.cpp:0:0
  intel#7 0x0000000004fa7d88 (anonymous namespace)::CHR::run() ControlHeightReduction.cpp:0:0
  intel#8 0x0000000004fa3618 llvm::ControlHeightReductionPass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (bin/opt+0x4fa3618)
```

Tests: opt <
llvm/test/Transforms/PGOProfile/chr-unknown-profdata-crash.ll
-passes='require<profile-summary>,function(chr)' -force-chr
-chr-merge-threshold=1 -disable-output

Author: peterwaller-arm

llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp

@@ -1992,8 +1992,8 @@ void CHR::addToMergedCondition(bool IsTrueBiased, Value *Cond,
 
   // Use logical and to avoid propagating poison from later conditions.
   MergedCondition = IRB.CreateLogicalAnd(MergedCondition, Cond);
-  setExplicitlyUnknownBranchWeightsIfProfiled(
-      *cast<Instruction>(MergedCondition), DEBUG_TYPE);
+  if (auto *MergedInst = dyn_cast<Instruction>(MergedCondition))
+    setExplicitlyUnknownBranchWeightsIfProfiled(*MergedInst, DEBUG_TYPE, &F);
 }
 
 void CHR::transformScopes(SmallVectorImpl<CHRScope *> &CHRScopes) {

llvm/test/Transforms/PGOProfile/chr-unknown-profdata-crash.ll

@@ -0,0 +1,33 @@
+; RUN: opt < %s -passes='require<profile-summary>,function(chr)' -force-chr -chr-merge-threshold=1 -disable-output
+
+; Regression test for a crash in CHR when setting unknown profdata on the
+; merged condition. IRBuilder::CreateLogicalAnd is implemented as a select and
+; can constant-fold to a non-Instruction value (e.g. `i1 true`). The buggy code
+; assumed it always produced an Instruction and did `cast<Instruction>(V)`,
+; which can segfault in release builds.
+define void @repro_crash() {
+entry:
+  br i1 true, label %then, label %exit, !prof !15
+
+then:
+  br label %exit
+
+exit:
+  ret void
+}
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 1, !"ProfileSummary", !1}
+!1 = !{!2, !3, !4, !5, !6, !7, !8, !9}
+!2 = !{!"ProfileFormat", !"InstrProf"}
+!3 = !{!"TotalCount", i64 10000}
+!4 = !{!"MaxCount", i64 10}
+!5 = !{!"MaxInternalCount", i64 1}
+!6 = !{!"MaxFunctionCount", i64 1000}
+!7 = !{!"NumCounts", i64 1}
+!8 = !{!"NumFunctions", i64 1}
+!9 = !{!"DetailedSummary", !10}
+!10 = !{!11}
+!11 = !{i32 999999, i64 1, i32 1}
+
+!15 = !{!"branch_weights", i32 100, i32 1}