[DebugInfo] Turn instruction referencing on by default for x86

This patch is designed to be reverted -- it activates a reasonably large
block of new-ish code, so some turbulence is likely.

Instruction referencing is best summarised, and it being on-by-default,
is discussed here:

    https://lists.llvm.org/pipermail/llvm-dev/2021-November/153653.html

Differential Revision: https://reviews.llvm.org/D114631

llvm/lib/CodeGen/CommandFlags.cpp

@@ -699,5 +699,7 @@ void codegen::setFunctionAttributes(StringRef CPU, StringRef Features,
 }
 
 bool codegen::getDefaultValueTrackingVariableLocations(const llvm::Triple &T) {
+  if (T.getArch() == llvm::Triple::x86_64)
+    return true;
   return false;
 }

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 it for pre-landing the patch, and to allow a minimal delta if it
-;; has to be reverted.
-; XFAIL: *
-
 ; INSTRREFON: DBG_INSTR_REF
 ; INSTRREFOFF: DBG_VALUE