pgsql: Inline initial comparisons in TestForOldSnapshot()

Inline initial comparisons in TestForOldSnapshot()

Even with old_snapshot_threshold = -1 (which disables the "snapshot
too old" feature), performance regressions were seen at moderate to
high concurrency.  For example, a one-socket, four-core system
running 200 connections at saturation could see up to a 2.3%
regression, with larger regressions possible on NUMA machines.
By inlining the early (smaller, faster) tests in the
TestForOldSnapshot() function, the i7 case dropped to a 0.2%
regression, which could easily just be noise, and is clearly an
improvement.  Further testing will show whether more is needed.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/11e178d0dc4bc2328ae4759090b3c48b07023fab

Modified Files
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src/backend/storage/buffer/bufmgr.c | 28 +++++-----------------------
src/include/storage/bufmgr.h        | 32 +++++++++++++++++++++++++++++++-
2 files changed, 36 insertions(+), 24 deletions(-)