Revert rG6a605b97a200 due to excessive memory use

Over in the comments for D116821, some use-cases have cropped up where
there's a substantial increase in memory usage. A quick inspection
shows that a) it's a lot of memory and b) there are several things to
be done to reduce it. Reverting (via disabling this feature by default)
to avoid bothering people in the meantime.

llvm/lib/CodeGen/LiveDebugValues/LiveDebugValues.cpp

@@ -124,10 +124,13 @@ bool LiveDebugValues::runOnMachineFunction(MachineFunction &MF) {
 
 bool llvm::debuginfoShouldUseDebugInstrRef(const Triple &T) {
   // Enable by default on x86_64, disable if explicitly turned off on cmdline.
+  // Disabled while https://reviews.llvm.org/D116821 is investigated.
+#if 0
   if (T.getArch() == llvm::Triple::x86_64 &&
       ValueTrackingVariableLocations != cl::boolOrDefault::BOU_FALSE)
     return true;
+#endif
 
-  // Otherwise: enable if explicitly requestedo n command line.
+  // Otherwise: enable if explicitly requested on command line.
   return ValueTrackingVariableLocations == cl::boolOrDefault::BOU_TRUE;
 }

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

@@ -13,6 +13,10 @@
 ;; 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