[DebugInfo][LoopUnroll] Preserve DebugLocs on optimized cond branches

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.

Author: SLTozer

llvm/lib/Transforms/Utils/LoopUnroll.cpp

@@ -883,7 +883,8 @@ 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]] = !{}
+;.