[DebugInfo] Variables with only empty values emitting when one is variadic

This patch fixes a simple bug in DbgValueHistoryMap::hasNonEmptyLocation
that caused it to treat empty DBG_VALUE_LIST instructions as non-empty
when determining whether to emit a variable or not.

Differential Revision: https://reviews.llvm.org/D133925

Author: SLTozer

llvm/lib/CodeGen/AsmPrinter/DbgEntityHistoryCalculator.cpp

@@ -264,7 +264,7 @@ bool DbgValueHistoryMap::hasNonEmptyLocation(const Entries &Entries) const {
     const MachineInstr *MI = Entry.getInstr();
     assert(MI->isDebugValue());
     // A DBG_VALUE $noreg is an empty variable location
-    if (MI->getOperand(0).isReg() && MI->getOperand(0).getReg() == 0)
+    if (MI->isUndefDebugValue())
       continue;
 
     return true;

llvm/test/DebugInfo/X86/dbg_value_list_emission.mir

@@ -93,9 +93,7 @@ body:             |
     
     ; (8) Check that a single $noreg location invalidates the entire entry.
     DBG_VALUE_LIST !30, !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_arg, 1, DW_OP_plus, DW_OP_stack_value), $eax, $noreg, debug-location !15
-    ; CHECK:      DW_TAG_variable
-    ; CHECK-NEXT:   DW_AT_name ("localh")
-    ; CHECK-NOT:   DW_AT_location
+    ; CHECK-NOT:   DW_AT_name ("localh")
 
     RET64 debug-location !15
 ...