[DebugInfo] replaceDbgUsesWithUndef after sroa promotes allocas

[Orlando]

See D73478

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

void PromoteMem2Reg::run() {
 
   // Remove the allocas themselves from the function.
   for (Instruction *A : Allocas) {
     // If there are any uses of the alloca instructions left, they must be in
     // unreachable basic blocks that were not processed by walking the dominator
     // tree. Just delete the users now.
     if (!A->use_empty())
       A->replaceAllUsesWith(UndefValue::get(A->getType()));
+    replaceDbgUsesWithUndef(A);
     A->eraseFromParent();
   }
 
   // Remove alloca's dbg.declare instrinsics from the function.
   for (auto &Declares : AllocaDbgDeclares)
     for (auto *DII : Declares)
       DII->eraseFromParent();
 

llvm/test/DebugInfo/X86/sroa-undef-dbg-value-4.ll

+; RUN: opt -sroa -S %s | FileCheck %s
+; CHECK: call void @llvm.dbg.value(metadata i16* undef, metadata ![[MD_c:[0-9]+]],
+; CHECK: ![[MD_c]] = !DILocalVariable(name: "c",
+
+; clang reduce.c -O0 -g -emit-llvm -S -Xclang -disable-O0-optnone -o -
+; cat reduce.c
+; void a() {
+;   short b = 3;
+;   short *c = &b;
+;   for (;;)
+;     *c = 0;
+; }
+
+define dso_local void @a() !dbg !7 {
+entry:
+  %b = alloca i16, align 2
+  %c = alloca i16*, align 8
+  call void @llvm.dbg.declare(metadata i16* %b, metadata !10, metadata !DIExpression()), !dbg !12
+  store i16 3, i16* %b, align 2, !dbg !12
+  call void @llvm.dbg.declare(metadata i16** %c, metadata !13, metadata !DIExpression()), !dbg !15
+  store i16* %b, i16** %c, align 8, !dbg !15
+  br label %for.cond, !dbg !16
+
+for.cond:                                         ; preds = %for.cond, %entry
+  %0 = load i16*, i16** %c, align 8, !dbg !17
+  store i16 0, i16* %0, align 2, !dbg !20
+  br label %for.cond, !dbg !21, !llvm.loop !22
+}
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "reduce.c", directory: "/")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 10.0.0"}
+!7 = distinct !DISubprogram(name: "a", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null}
+!10 = !DILocalVariable(name: "b", scope: !7, file: !1, line: 2, type: !11)
+!11 = !DIBasicType(name: "short", size: 16, encoding: DW_ATE_signed)
+!12 = !DILocation(line: 2, column: 9, scope: !7)
+!13 = !DILocalVariable(name: "c", scope: !7, file: !1, line: 3, type: !14)
+!14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !11, size: 64)
+!15 = !DILocation(line: 3, column: 10, scope: !7)
+!16 = !DILocation(line: 4, column: 3, scope: !7)
+!17 = !DILocation(line: 5, column: 6, scope: !18)
+!18 = distinct !DILexicalBlock(scope: !19, file: !1, line: 4, column: 3)
+!19 = distinct !DILexicalBlock(scope: !7, file: !1, line: 4, column: 3)
+!20 = !DILocation(line: 5, column: 8, scope: !18)
+!21 = !DILocation(line: 4, column: 3, scope: !18)
+!22 = distinct !{!22, !23, !24}
+!23 = !DILocation(line: 4, column: 3, scope: !19)
+!24 = !DILocation(line: 5, column: 10, scope: !19)