Re: optimize lookups in snapshot [sub]xip arrays

Here is a new patch set.  0001 is the currently-proposed patch from the
other thread [0] for determining SSE2 support.  0002 introduces the
optimized linear search function.  And 0003 makes use of the new function
for the [sub]xip lookups in XidInMVCCSnapshot().

[0] https://postgr.es/m/CAFBsxsGktHL7%3DJXbgnKTi_uL0VRPcH4FSAqc6yK-3%2BJYfqPPjA%40mail.gmail.com

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Nathan Bossart
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