[llvm] [RegAllocFast] Give an undef tied use the register of its tied def (PR #222249)

[Fangrui Song via llvm-commits]

================
@@ -1031,6 +1031,28 @@ void RegAllocFastImpl::allocVirtRegUndef(MachineOperand &MO) {
   if (!shouldAllocateRegister(VirtReg))
     return;
 
+  // The def is freed before the uses are allocated, so a tie is the only
----------------
MaskRay wrote:

I'm preparing ` [CodeGen] Reject an undef tied use sharing a virtual register with another read` to ban ` %1:gr64 = XOR64rr undef %0:gr64, undef %0:gr64, implicit-def dead $eflags` so that we can simplify this approach

https://github.com/llvm/llvm-project/pull/222249