model: optimize get_last_conflict() search

model: optimize get_last_conflict() search

Relaxed operations should never require backtracking, so don't even
bother with searching the list in such cases. I ran some simple
benchmarking tests with linuxrwlocks (timing results below). Also, perf
shows a significant decrease in % time spent in
ModelAction::could_synchronize_with.

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Before:
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$ time ./run.sh test/linuxrwlocks.o -f 10 -m 10
Number of complete, bug-free executions: 7110
Number of redundant executions: 29
Number of buggy executions: 0
Number of infeasible executions: 43578
Total executions: 50717
Total nodes created: 63463

real	0m7.274s
user	0m6.596s
sys	0m0.652s

2.10%  linuxrwlocks.o  libmodel.so          [.] ModelAction::could_synchronize_with(ModelAction const*) const

------------
After:
------------

$ time ./run.sh test/linuxrwlocks.o -f 10 -m 10
Number of complete, bug-free executions: 7110
Number of redundant executions: 29
Number of buggy executions: 0
Number of infeasible executions: 43578
Total executions: 50717
Total nodes created: 63463

real	0m6.690s
user	0m6.000s
sys	0m0.664s

0.73%  linuxrwlocks.o  libmodel.so          [.] ModelAction::could_synchronize_with(ModelAction const*) const