[cxx-mem-model] Consolidation 2 - the fetch ops.

[Andrew MacLeod]

Here is the second part of the consolidation patch, the chunks handling 
add, sub, and, xor, and or.

Along the way , I also discovered the old __sync routines also had a 
NAND operation.  It makes consolidation easier to just add the operation 
to the __sync_mem routines as well, even though c++ won't need it.

There were actually 3 different rtl patterns which may be used to 
implement an operation:
   sync_op  		if the result is not used.
   sync_fetch_and_op 	if returning the pre-operation value.
   sync_op_and_fetch 	if returning the post-operation value.

There are now 3 new variations of those which take a memory model 
parameter.  sync_mem_op_fetch and sync_mem_fetch_op were already 
implemented, but I also added 'sync_mem_op' in this patch.

The old expansion code has been assimilated in the new expander, and I 
think its more manageable now. The new expansion routine consecutively 
tries each of the 6 patterns in order from best match to worst, until it 
either generates an instruction or falls through to a compare_and_swap loop.

A single new function 'maybe_emit_op' is a common entry point for 
attempting each of the 6 patterns.

expand_builtin_sync_operation was being passed a superfluous 'ignore' 
parameter to indicate the result is ignored. The expanders check if the 
target is const0_rtx for that condition.

The 5 fetch_op testcases were expanded to test the new NAND variation as 
well as all the variations with an ignored result.

Now in order to enable a port for memory model builtins, we don't need 
to create yet another set of patterns. We only have to port the old sync 
patterns (even partially if desired) to take an extra parameter and all 
the old and new routines will still behave correctly. If we get to the 
point where the old patterns have been converted on all ports, we can 
actually remove internal support for the old patterns. (the old 
built-ins will likely remain for compatibility, but simply map to the 
new patterns)

Bootstrapped and tested on x86_64-unknown-linux-gnu.

Andrew
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