pgsql: Micro optimize LWLockAttemptLock() a bit.

Micro optimize LWLockAttemptLock() a bit.

LWLockAttemptLock pointlessly read the lock's state in every loop
iteration, even though pg_atomic_compare_exchange_u32() returns the old
value. Instead do that only once before the loop iteration.

Additionally there's no need to have the expected_state variable,
old_state mostly had the same value anyway.

Noticed-By: Heikki Linnakangas
Backpatch: 9.5, no reason to let the branches diverge at this point

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a4b09af3e93707351f1d4b8efafabfa135c0f70b

Modified Files
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src/backend/storage/lmgr/lwlock.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)