[DebugInfo] Re-enable instruction referencing for x86_64

After discussion in D116821 this was turned off in 74db5c8c95,
14aaaa1236 applied to limit the maximum memory consumption in rare
conditions, plus some performance patches.

llvm/lib/CodeGen/LiveDebugValues/LiveDebugValues.cpp

@@ -123,6 +123,11 @@ bool LiveDebugValues::runOnMachineFunction(MachineFunction &MF) {
 }
 
 bool llvm::debuginfoShouldUseDebugInstrRef(const Triple &T) {
+  // Enable by default on x86_64, disable if explicitly turned off on cmdline.
+  if (T.getArch() == llvm::Triple::x86_64 &&
+      ValueTrackingVariableLocations != cl::boolOrDefault::BOU_FALSE)
+    return true;
+
   // Enable if explicitly requested on command line.
   return ValueTrackingVariableLocations == cl::boolOrDefault::BOU_TRUE;
 }

llvm/test/DebugInfo/X86/instr-ref-flag.ll

@@ -13,10 +13,6 @@
 ;; by llc by default, and that it can be turned explicitly on or off as
 ;; desired.
 
-;; Xfail due to faults found in the discussion on
-;; https://reviews.llvm.org/D116821
-; XFAIL: *
-
 ; INSTRREFON: DBG_INSTR_REF
 ; INSTRREFOFF: DBG_VALUE