[CSSPGO] Fix the issue of preinliner import function list (#85719)

By design, when the nested profile is pre-inliner based, we should fully
honor pre-inliner decision, fix it by setting threshold to zero. We
observed a perf win on one internal service, no negative impact for
other big services.

llvm/lib/Transforms/IPO/SampleProfile.cpp

@@ -1167,6 +1167,9 @@ void SampleProfileLoader::findExternalInlineCandidate(
   // For AutoFDO profile, retrieve candidate profiles by walking over
   // the nested inlinee profiles.
   if (!FunctionSamples::ProfileIsCS) {
+    // Set threshold to zero to honor pre-inliner decision.
+    if (UsePreInlinerDecision)
+      Threshold = 0;
     Samples->findInlinedFunctions(InlinedGUIDs, SymbolMap, Threshold);
     return;
   }

llvm/test/Transforms/SampleProfile/Inputs/csspgo-import-list-preinliner.prof

@@ -0,0 +1,14 @@
+main:8001:0
+ 1: 0
+ 2: 2000
+ 3: 2000
+ 4: 0
+ 5: 2000
+ 6: 2000
+ 7: 0
+ 8: 0
+ 9: bar:1
+  1: 1
+  !CFGChecksum: 4294967295
+  !Attributes: 2
+ !CFGChecksum: 563088156202820

llvm/test/Transforms/SampleProfile/csspgo-import-list-preinliner.ll

@@ -0,0 +1,50 @@
+; RUN: opt < %s -passes='thinlto-pre-link<O2>' -pgo-kind=pgo-sample-use-pipeline -sample-profile-file=%S/Inputs/csspgo-import-list-preinliner.prof -S -profile-summary-cutoff-hot=100000 -sample-profile-use-preinliner=0 | FileCheck %s --check-prefix=DISABLE-PREINLINE
+; RUN: opt < %s -passes='thinlto-pre-link<O2>' -pgo-kind=pgo-sample-use-pipeline -sample-profile-file=%S/Inputs/csspgo-import-list-preinliner.prof -S -profile-summary-cutoff-hot=100000 | FileCheck %s
+
+; The GUID of bar is -2012135647395072713
+
+; DISABLE-PREINLINE-NOT: -2012135647395072713
+; CHECK: [[#]] = !{!"function_entry_count", i64 1, i64 -2012135647395072713}
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @main() #0 {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond, %entry
+  call void @llvm.pseudoprobe(i64 0, i64 0, i32 0, i64 0)
+  %call2 = call i32 @bar(), !dbg !9
+  br label %for.cond
+}
+
+declare i32 @bar()
+
+; Function Attrs: nocallback nofree nosync nounwind willreturn memory(inaccessiblemem: readwrite)
+declare void @llvm.pseudoprobe(i64, i64, i32, i64) #1
+
+attributes #0 = { "use-sample-profile" }
+attributes #1 = { nocallback nofree nosync nounwind willreturn memory(inaccessiblemem: readwrite) }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!7}
+!llvm.pseudo_probe_desc = !{!8}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 19.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/home/", checksumkind: CSK_MD5, checksum: "1bff37d8b3f7858b0bc29ab4efdf9422")
+!2 = !{!3}
+!3 = !DIGlobalVariableExpression(var: !4, expr: !DIExpression())
+!4 = distinct !DIGlobalVariable(name: "x", scope: !0, file: !1, line: 2, type: !5, isLocal: false, isDefinition: true)
+!5 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !6)
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i64 -2624081020897602054, i64 563108639284859, !"main"}
+!9 = !DILocation(line: 11, column: 10, scope: !10)
+!10 = !DILexicalBlockFile(scope: !11, file: !1, discriminator: 186646615)
+!11 = distinct !DILexicalBlock(scope: !12, file: !1, line: 8, column: 40)
+!12 = distinct !DILexicalBlock(scope: !13, file: !1, line: 8, column: 3)
+!13 = distinct !DILexicalBlock(scope: !14, file: !1, line: 8, column: 3)
+!14 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 6, type: !15, scopeLine: 7, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !16)
+!15 = distinct !DISubroutineType(types: !16)
+!16 = !{}