[DebugInfo][LoopUnroll] Preserve DebugLocs on optimized cond branches (#114225)

This patch fixes a simple error where as part of loop unrolling we
optimize conditional loop-exiting branches into unconditional branches
when we know that they will or won't exit the loop, but does not
propagate the source location of the original branch to the new one.

Found using #107279.

Author: SLTozer

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -880,7 +880,8 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     DeadSucc->removePredecessor(Src, /* KeepOneInputPHIs */ true);
 
     // Replace the conditional branch with an unconditional one.
-    BranchInst::Create(Dest, Term->getIterator());
+    auto *BI = BranchInst::Create(Dest, Term->getIterator());
+    BI->setDebugLoc(Term->getDebugLoc());
     Term->eraseFromParent();
 
     DTUpdates.emplace_back(DominatorTree::Delete, Src, DeadSucc);

llvm/test/Transforms/LoopUnroll/preserve-branch-debuglocs.ll

@@ -0,0 +1,52 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals all --version 5
+; RUN: opt < %s -passes=loop-unroll -S | FileCheck %s
+; RUN: opt < %s -passes=loop-unroll-full -S | FileCheck %s
+
+;; Loop Unrolling should preserve the DebugLocs of conditional branches that get
+;; optimized into unconditional branches.
+
+define i32 @_ZeqRK4int3S1_() {
+; CHECK-LABEL: define i32 @_ZeqRK4int3S1_() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
+; CHECK:       [[FOR_COND:.*]]:
+; CHECK-NEXT:    br label %[[CLEANUP:.*]], !dbg [[DBG3:![0-9]+]]
+; CHECK:       [[FOR_BODY]]:
+; CHECK-NEXT:    br i1 false, label %[[FOR_COND]], label %[[CLEANUP]]
+; CHECK:       [[CLEANUP]]:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  br label %for.body
+
+for.cond:                                         ; preds = %for.body
+  br i1 false, label %cleanup, label %for.body, !dbg !3
+
+for.body:                                         ; preds = %for.cond, %entry
+  br i1 false, label %for.cond, label %cleanup
+
+cleanup:                                          ; preds = %for.body, %for.cond
+  ret i32 0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 20.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug)
+!1 = !DIFile(filename: "test.cpp", directory: "/tmp")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !DILocation(line: 304, column: 2, scope: !4)
+!4 = distinct !DILexicalBlock(scope: !5, file: !1, line: 304, column: 2)
+!5 = distinct !DISubprogram(name: "operator==", linkageName: "_ZeqRK4int3S1_", scope: !1, file: !1, line: 302, type: !6, scopeLine: 303, spFlags: DISPFlagDefinition, unit: !0)
+!6 = distinct !DISubroutineType(types: !7)
+!7 = !{}
+;.
+; CHECK: [[META0:![0-9]+]] = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: [[META1:![0-9]+]], producer: "{{.*}}clang version {{.*}}", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug)
+; CHECK: [[META1]] = !DIFile(filename: "test.cpp", directory: {{.*}})
+; CHECK: [[META2:![0-9]+]] = !{i32 2, !"Debug Info Version", i32 3}
+; CHECK: [[DBG3]] = !DILocation(line: 304, column: 2, scope: [[META4:![0-9]+]])
+; CHECK: [[META4]] = distinct !DILexicalBlock(scope: [[META5:![0-9]+]], file: [[META1]], line: 304, column: 2)
+; CHECK: [[META5]] = distinct !DISubprogram(name: "operator==", linkageName: "_ZeqRK4int3S1_", scope: [[META1]], file: [[META1]], line: 302, type: [[META6:![0-9]+]], scopeLine: 303, spFlags: DISPFlagDefinition, unit: [[META0]])
+; CHECK: [[META6]] = distinct !DISubroutineType(types: [[META7:![0-9]+]])
+; CHECK: [[META7]] = !{}
+;.