[DebugInfo][LoopStrengthReduce] Fix missing debug location updates (#97519)

Fix #97510 .

Note that, for the new phi instruction `NewPH`, which replaces the old
phi `PH` and the cast `ShadowUse`, I choose to propagate the debug
location of `PH` to it, because the cast is eliminated according to the
optimization semantics.

Author: Apochens

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -2404,13 +2404,15 @@ void LSRInstance::OptimizeShadowIV() {
 
     /* Add new PHINode. */
     PHINode *NewPH = PHINode::Create(DestTy, 2, "IV.S.", PH->getIterator());
+    NewPH->setDebugLoc(PH->getDebugLoc());
 
     /* create new increment. '++d' in above example. */
     Constant *CFP = ConstantFP::get(DestTy, C->getZExtValue());
     BinaryOperator *NewIncr = BinaryOperator::Create(
         Incr->getOpcode() == Instruction::Add ? Instruction::FAdd
                                               : Instruction::FSub,
         NewPH, CFP, "IV.S.next.", Incr->getIterator());
+    NewIncr->setDebugLoc(Incr->getDebugLoc());
 
     NewPH->addIncoming(NewInit, PH->getIncomingBlock(Entry));
     NewPH->addIncoming(NewIncr, PH->getIncomingBlock(Latch));

llvm/test/Transforms/LoopStrengthReduce/X86/preserving-debugloc-phi-binop.ll

@@ -0,0 +1,59 @@
+; RUN: opt -S -passes=loop-reduce -mtriple=x86_64-unknown-unknown < %s | FileCheck %s
+
+; Check that LoopStrengthReduce's OptimizeShadowIV() propagates the debug
+; locations of the old phi (`%accum`) and binop (`%accum.next`) instruction
+; to the new phi and binop instruction, respectively.
+
+target datalayout = "n8:16:32:64"
+
+define i32 @foobar6() !dbg !5 {
+; CHECK-LABEL: define i32 @foobar6(
+; CHECK:  loop:
+; CHECK:    [[IV_S_:%.*]] = phi double [ -3.220000e+03, %[[ENTRY:.*]] ], [ [[IV_S_NEXT_:%.*]], %loop ], !dbg [[DBG9:![0-9]+]]
+; CHECK:    [[IV_S_NEXT_]] = fadd double [[IV_S_]], 0x41624E65A0000000, !dbg [[DBG11:![0-9]+]]
+; CHECK:  exit:
+;
+entry:
+  br label %loop, !dbg !8
+
+loop:                                             ; preds = %loop, %entry
+  %accum = phi i32 [ -3220, %entry ], [ %accum.next, %loop ], !dbg !9
+  %iv = phi i32 [ 12, %entry ], [ %iv.next, %loop ], !dbg !10
+  %tmp1 = sitofp i32 %accum to double, !dbg !11
+  tail call void @foo(double %tmp1), !dbg !12
+  %accum.next = add nsw i32 %accum, 9597741, !dbg !13
+  %iv.next = add nuw nsw i32 %iv, 1, !dbg !14
+  %exitcond = icmp ugt i32 %iv, 235, !dbg !15
+  br i1 %exitcond, label %exit, label %loop, !dbg !16
+
+exit:                                             ; preds = %loop
+  ret i32 %accum.next, !dbg !17
+}
+
+declare void @foo(double)
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!2, !3}
+!llvm.module.flags = !{!4}
+
+; CHECK: [[DBG9]] = !DILocation(line: 2,
+; CHECK: [[DBG11]] = !DILocation(line: 6,
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "main.ll", directory: "/")
+!2 = !{i32 10}
+!3 = !{i32 0}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foobar6", linkageName: "foobar6", scope: null, file: !1, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!6 = !DISubroutineType(types: !7)
+!7 = !{}
+!8 = !DILocation(line: 1, column: 1, scope: !5)
+!9 = !DILocation(line: 2, column: 1, scope: !5)
+!10 = !DILocation(line: 3, column: 1, scope: !5)
+!11 = !DILocation(line: 4, column: 1, scope: !5)
+!12 = !DILocation(line: 5, column: 1, scope: !5)
+!13 = !DILocation(line: 6, column: 1, scope: !5)
+!14 = !DILocation(line: 7, column: 1, scope: !5)
+!15 = !DILocation(line: 8, column: 1, scope: !5)
+!16 = !DILocation(line: 9, column: 1, scope: !5)
+!17 = !DILocation(line: 10, column: 1, scope: !5)