Re-commit r319490 "XOR the frame pointer with the stack cookie when protecting the stack"

The patch originally broke Chromium (crbug.com/791714) due to its failing to
specify that the new pseudo instructions clobber EFLAGS. This commit fixes
that.

> Summary: This strengthens the guard and matches MSVC.
>
> Reviewers: hans, etienneb
>
> Subscribers: hiraditya, JDevlieghere, vlad.tsyrklevich, llvm-commits
>
> Differential Revision: https://reviews.llvm.org/D40622

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319824 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/StackProtector.cpp

@@ -385,8 +385,12 @@ static bool CreatePrologue(Function *F, Module *M, ReturnInst *RI,
 ///  - The epilogue checks the value stored in the prologue against the original
 ///    value. It calls __stack_chk_fail if they differ.
 bool StackProtector::InsertStackProtectors() {
+  // If the target wants to XOR the frame pointer into the guard value, it's
+  // impossible to emit the check in IR, so the target *must* support stack
+  // protection in SDAG.
   bool SupportsSelectionDAGSP =
-      EnableSelectionDAGSP && !TM->Options.EnableFastISel;
+      TLI->useStackGuardXorFP() ||
+      (EnableSelectionDAGSP && !TM->Options.EnableFastISel);
   AllocaInst *AI = nullptr;       // Place on stack that stores the stack guard.
 
   for (Function::iterator I = F->begin(), E = F->end(); I != E;) {