[BranchFolding] Follow up #149999 crash fix

fbf6271 introduced an assertion failure as
setDebugValueUndef was called on DBG_LABELs, which isn't allowed and doesn't
make sense. Fix by skipping the call for DBG_LABELs and hoisting, in line with
the original behaviour.

Author: OCHyams

llvm/lib/CodeGen/BranchFolding.cpp

@@ -2103,8 +2103,9 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
         DI->eraseFromParent();
         return;
       }
-
-      DI->setDebugValueUndef();
+      // Move DBG_LABELs without modifying them. Set DBG_VALUEs undef.
+      if (!DI->isDebugLabel())
+        DI->setDebugValueUndef();
       DI->moveBefore(&*Loc);
     };
 

llvm/test/DebugInfo/X86/branch-folder-dbg.mir

@@ -9,11 +9,15 @@
 ## can be killed.
 ##
 ## Check DBG_PHIs are deleted rather than hoisted (implicit-check-not).
+##
+## Check DBG_LABELs are hoisted and not modified (and don't cause a crash).
 
 # CHECK: bb.0
 # CHECK:      CALL64pcrel32 @f, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
 ## --- Start splice from bb.2.if.else (and debug instructions from bb.1.if.then) ---
+# CHECK-NEXT: DBG_LABEL 0
 # CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(),  debug-___location ![[#]]
+# CHECK-NEXT: DBG_LABEL 1
 # CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(),  debug-___location ![[#]]
 # CHECK-NEXT: $edi = MOV32r0 implicit-def dead $eflags, debug-instr-number 2, debug-___location !DILocation(line: 0, scope: ![[#]])
 # CHECK-NEXT: DBG_VALUE $noreg, $noreg, ![[#]], !DIExpression(DW_OP_LLVM_arg, 0),  debug-___location ![[#]]
@@ -98,6 +102,7 @@ body:             |
     successors: %bb.3(0x80000000)
 
     DBG_PHI $esp, 3
+    DBG_LABEL 0
     DBG_VALUE $esi, $noreg, !11, !DIExpression(),  debug-___location !13
     $edi = MOV32r0 implicit-def dead $eflags, debug-instr-number 1, debug-___location !14
     DBG_INSTR_REF !11, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(1, 0),  debug-___location !13
@@ -109,6 +114,7 @@ body:             |
     successors: %bb.3(0x80000000)
 
     DBG_PHI $esp, 4
+    DBG_LABEL 1
     DBG_VALUE $esp, $noreg, !11, !DIExpression(), debug-___location !13
     $edi = MOV32r0 implicit-def dead $eflags, debug-instr-number 2, debug-___location !16
     DBG_INSTR_REF !11, !DIExpression(DW_OP_LLVM_arg, 0), dbg-instr-ref(2, 0),  debug-___location !13