[X86][AMX] Fix a bug after llvm#83628

We need to check if `GR64Cand` a valid register before using it.

Test is not needed since it's covered in llvm-test-suite.

Fixes llvm#90954

Author: phoebewang

llvm/lib/Target/X86/X86LowerTileCopy.cpp

@@ -146,7 +146,7 @@ bool X86LowerTileCopy::runOnMachineFunction(MachineFunction &MF) {
           addFrameReference(BuildMI(MBB, MI, DL, TII->get(Opc)), TileSS)
               .addReg(SrcReg, getKillRegState(SrcMO.isKill()));
       MachineOperand &MO = NewMI->getOperand(2);
-      MO.setReg(GR64Cand);
+      MO.setReg(GR64Cand ? GR64Cand : X86::RAX);
       MO.setIsKill(true);
       // tileloadd (%sp, %idx), %tmm
       Opc = GET_EGPR_IF_ENABLED(X86::TILELOADD);
@@ -157,7 +157,7 @@ bool X86LowerTileCopy::runOnMachineFunction(MachineFunction &MF) {
         // restore %rax
         // mov (%sp) %rax
         addFrameReference(
-            BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm), GR64Cand), StrideSS);
+            BuildMI(MBB, MI, DL, TII->get(X86::MOV64rm), X86::RAX), StrideSS);
       }
       MI.eraseFromParent();
       Changed = true;