[AArch64][CGP] Sink zext feeding stxr/stlxr into the same block.

The usual CodeGenPrepare trickery, on a target-specific intrinsic.
Without this, the expansion of atomics will usually have the zext
be hoisted out of the loop, defeating the various patterns we have
to catch this precise case.

Differential Revision: http://reviews.llvm.org/D9930

llvm-svn: 238054