[X86] Fix a warning

This patch fixes:

  llvm/lib/Target/X86/X86ISelLowering.cpp:31886:11: error: unused
  variable 'M' [-Werror,-Wunused-variable]

(cherry picked from commit f83eeac)

Author: kazutakahirata

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -31766,7 +31766,6 @@ X86TargetLowering::lowerIdempotentRMWIntoFencedLoad(AtomicRMWInst *AI) const {
 
   IRBuilder<> Builder(AI);
   Builder.CollectMetadataToCopy(AI, {LLVMContext::MD_pcsections});
-  Module *M = Builder.GetInsertBlock()->getParent()->getParent();
   auto SSID = AI->getSyncScopeID();
   // We must restrict the ordering to avoid generating loads with Release or
   // ReleaseAcquire orderings.