Rename AtomicExpandLoadLinked into AtomicExpand

AtomicExpandLoadLinked is currently rather ARM-specific. This patch is the first of
a group that aim at making it more target-independent. See
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-August/075873.html
for details

The command line option is "atomic-expand"

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216231 91177308-0d34-0410-b5e6-96231b3b80d8