[PPC] Remove isBarrier from CFENCE8's definition.

Summary:
This is my misunderstanding on isBarrier. It's not for memory barriers,
but for other control flow purposes. lwsync doesn't have it either.

This fixes a simple crash with -verify-machineinstrs like below:

  define void @Foo() {
  entry:
    %tmp = load atomic i64, i64* undef acquire, align 8
    unreachable
  }

I deliberately don't want to check in the test, since there is little
chance to regress on such a mistake. Such a test adds noise to the code
base.

I plan to check in first, since it fixes a crash, and the fix is obvious.

Reviewers: kbarton, echristo

Subscribers: sanjoy, nemanjai, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D34314

llvm-svn: 305624