ARM: sink atomic release barrier as far as possible into cmpxchg.

DMB instructions can be expensive, so it's best to avoid them if possible. In
atomicrmw operations there will always be an attempted store so a release
barrier is always needed, but in the cmpxchg case we can delay the DMB until we
know we'll definitely try to perform a store (and so need release semantics).

In the strong cmpxchg case this isn't quite free: we must duplicate the LDREX
instructions to skip the barrier on subsequent iterations. The basic outline
becomes:

        ldrex rOld, [rAddr]
        cmp rOld, rDesired
        bne Ldone
        dmb
    Lloop:
        strex rRes, rNew, [rAddr]
        cbz rRes Ldone
        ldrex rOld, [rAddr]
        cmp rOld, rDesired
        beq Lloop
    Ldone:

So we'll skip this version for strong operations in "minsize" functions.

llvm-svn: 261568