[X86] Check if machine loop is passed while getting loop alignment (#77283)

After d6bb96e, calling
getPrefLoopAlignment without passing in a pointer to a MachineLoop
causes a segmentation fault. This conflicts with the API in
TargetLoweringBase where the default MachineLoop pointer passed is
nullptr. This patch fixes this by checking if the pointer points to
something before enabling the optional functionality.

Author: boomanaiden154

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -57672,7 +57672,7 @@ X86TargetLowering::getStackProbeSize(const MachineFunction &MF) const {
 }
 
 Align X86TargetLowering::getPrefLoopAlignment(MachineLoop *ML) const {
-  if (ML->isInnermost() &&
+  if (ML && ML->isInnermost() &&
       ExperimentalPrefInnermostLoopAlignment.getNumOccurrences())
     return Align(1ULL << ExperimentalPrefInnermostLoopAlignment);
   return TargetLowering::getPrefLoopAlignment();