[SystemZ] Remove unnecessary serialization before volatile loads

This reverts the use of TargetLowering::prepareVolatileOrAtomicLoad
introduced by r196905.  Nothing in the semantics of the "volatile"
keyword or the definition of the z/Architecture actually requires
that volatile loads are preceded by a serialization operation, and
no other compiler on the platform actually implements this.

Since we've now seen a use case where this additional serialization
causes noticable performance degradation, this patch removes it.

The patch still leaves in the serialization before atomic loads,
which is now implemented directly in lowerATOMIC_LOAD.  (This also
seems overkill, but that can be addressed separately.)

llvm-svn: 306117