[DebugInfo] Re-enable instruction referencing for x86_64

After discussion in D116821 this was turned off in 74db5c8,
14aaaa1 applied to limit the maximum memory consumption in rare
conditions, plus some performance patches.

(cherry picked from commit 6e03a68)

Author: jmorse

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