IR: add "cmpxchg weak" variant to support permitted failure.

This commit adds a weak variant of the cmpxchg operation, as described
in C++11. A cmpxchg instruction with this modifier is permitted to
fail to store, even if the comparison indicated it should.

As a result, cmpxchg instructions must return a flag indicating
success in addition to their original iN value loaded. Thus, for
uniformity *all* cmpxchg instructions now return "{ iN, i1 }". The
second flag is 1 when the store succeeded.

At the DAG level, a new ATOMIC_CMP_SWAP_WITH_SUCCESS node has been
added as the natural representation for the new cmpxchg instructions.
It is a strong cmpxchg.

By default this gets Expanded to the existing ATOMIC_CMP_SWAP during
Legalization, so existing backends should see no change in behaviour.
If they wish to deal with the enhanced node instead, they can call
setOperationAction on it. Beware: as a node with 2 results, it cannot
be selected from TableGen.

Currently, no use is made of the extra information provided in this
patch. Test updates are almost entirely adapting the input IR to the
new scheme.

Summary for out of tree users:
------------------------------

+ Legacy Bitcode files are upgraded during read.
+ Legacy assembly IR files will be invalid.
+ Front-ends must adapt to different type for "cmpxchg".
+ Backends should be unaffected by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210903 91177308-0d34-0410-b5e6-96231b3b80d8
Docs: fix grammar error in description

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210904 91177308-0d34-0410-b5e6-96231b3b80d8
Docs: remove extra {} around result types.

It makes the types look like they're single-element structures. And
when we have instructions that *do* result in a struct, that can get
confusing rather quickly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210905 91177308-0d34-0410-b5e6-96231b3b80d8
[mips][mips64r6] Relocation R_MIPS_PC18_S3
Differential Revision: http://reviews.llvm.org/D3890


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210908 91177308-0d34-0410-b5e6-96231b3b80d8