[X86] Skip debug instructions in tile copy lowering (#209640)

X86LowerTileCopy iterates over instructions in reverse to track live
registers, calling LiveRegUnits::stepBackward on every instruction
unconditionally. Since commit 16b2ef3 added an assertion that
stepBackward must not receive debug instructions, functions containing
debug info hit the assertion during tile copy lowering.
This patch skips debug instructions early in the loop, matching the
guard pattern used in RegisterScavenging.cpp and X86FixupBWInsts.cpp.

Fixes #209512

(cherry picked from commit 3e4160f)

Author: Kristianerik

llvm/lib/Target/X86/X86LowerTileCopy.cpp

@@ -87,6 +87,8 @@ static bool lowerTileCopy(MachineFunction &MF) {
     LiveRegUnits UsedRegs(*TRI);
     UsedRegs.addLiveOuts(MBB);
     for (MachineInstr &MI : llvm::make_early_inc_range(reverse(MBB))) {
+      if (MI.isDebugInstr())
+        continue;
       UsedRegs.stepBackward(MI);
       if (!MI.isCopy())
         continue;

llvm/test/CodeGen/X86/AMX/pr209512.ll

@@ -0,0 +1,28 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+amx-int8 -mattr=+avx512f -verify-machineinstrs -o /dev/null
+
+define void @test_debug_instr(ptr %arg) #0 !dbg !4 {
+  #dbg_declare(ptr %arg, !12, !DIExpression(), !15)
+  %1 = call x86_amx @llvm.x86.cast.vector.to.tile.v1024i8(<1024 x i8> zeroinitializer)
+  %2 = call x86_amx @llvm.x86.tdpbssd.internal(i16 0, i16 0, i16 0, x86_amx %1, x86_amx %1, x86_amx %1)
+  ret void
+}
+
+declare x86_amx @llvm.x86.cast.vector.to.tile.v1024i8(<1024 x i8>) #1
+declare x86_amx @llvm.x86.tdpbssd.internal(i16, i16, i16, x86_amx, x86_amx, x86_amx) #2
+
+attributes #0 = { "target-features"="+amx-int8" }
+attributes #1 = { nocallback nofree nosync nounwind willreturn memory(none) }
+attributes #2 = { nounwind }
+
+!llvm.module.flags = !{!0}
+!llvm.dbg.cu = !{!1}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = distinct !DICompileUnit(language: DW_LANG_C11, file: !2, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, nameTableKind: None)
+!2 = !DIFile(filename: "test.c", directory: "/tmp")
+!3 = !{}
+!4 = distinct !DISubprogram(name: "test_debug_instr", linkageName: "test_debug_instr", scope: !2, file: !2, line: 1, type: !9, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !1, retainedNodes: !3)
+!9 = distinct !DISubroutineType(types: !3)
+!12 = distinct !DILocalVariable(name: "a", arg: 1, scope: !4, file: !2, line: 1, type: !13)
+!13 = !DICompositeType(tag: DW_TAG_structure_type, name: "__tile1024i", scope: !4, file: !2, size: 16384, align: 8192, flags: DIFlagPublic, elements: !3, identifier: "__tile1024i")
+!15 = !DILocation(line: 1, column: 1, scope: !4)