-Revert parts of 84326 and 84411. Distinquishing between fixed and non-fixed

stack slots and giving them different PseudoSourceValue's did not fix the
problem of post-alloc scheduling miscompiling llvm itself.
- Apply Dan's conservative workaround by assuming any non fixed stack slots can
alias other memory locations. This means a load from spill slot #1 cannot 
move above a store of spill slot #2. 
- Enable post-alloc scheduling for x86 at optimization leverl Default and above.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84424 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ScheduleDAGInstrs.cpp

@@ -106,10 +106,19 @@ static const Value *getUnderlyingObjectForInstr(const MachineInstr *MI) {
     return 0;
 
   V = getUnderlyingObject(V);
-  if (!isa<PseudoSourceValue>(V) && !isIdentifiedObject(V))
-    return 0;
+  if (const PseudoSourceValue *PSV = dyn_cast<PseudoSourceValue>(V)) {
+    // For now, ignore PseudoSourceValues which may alias LLVM IR values
+    // because the code that uses this function has no way to cope with
+    // such aliases.
+    if (PSV->isAliased())
+      return 0;
+    return V;
+  }
 
-  return V;
+  if (isIdentifiedObject(V))
+    return V;
+
+  return 0;
 }
 
 void ScheduleDAGInstrs::StartBlock(MachineBasicBlock *BB) {