Add a target-specific DAG combine on X86 to fold the common pattern of

Add a target-specific DAG combine on X86 to fold the common pattern of
fence-atomic-fence down to just the atomic op.  This is possible thanks to
X86's relatively strong memory model, which guarantees that locked instructions
(which are used to implement atomics) are implicit fences.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74435 91177308-0d34-0410-b5e6-96231b3b80d8