[AArch64] Emit clrex in the expanded cmpxchg fail block.

[AArch64] Emit clrex in the expanded cmpxchg fail block.

In the comparison failure block of a cmpxchg expansion, the initial
ldrex/ldxr will not be followed by a matching strex/stxr.
On ARM/AArch64, this unnecessarily ties up the execution monitor,
which might have a negative performance impact on some uarchs.

Instead, release the monitor in the failure block.
The clrex instruction was designed for this: use it.

Also see ARMARM v8-A B2.10.2:
"Exclusive access instructions and Shareable memory locations".

Differential Revision: http://reviews.llvm.org/D13033

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248291 91177308-0d34-0410-b5e6-96231b3b80d8