Enhance the query planner to enable it to use an index for IS NOT NULL

constraints, even if STAT4 is not enabled.

FossilOrigin-Name: 7b2a7c7314d2239992dc1d707280f3b75ba75bb074ba1e77b55cad01ca7fd51b

diff --cc manifest
index 0cc63599965ad592a79cacd06815a34a09d8e88a,8eeac2760fa1f465b4033117f649903db491bebc..192c496e180a02cb8a7d866bbd5893657de02704
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Do\snot\sallow\sVACUUM\sto\sresize\sthe\spage_size\sto\s512\sif\sthe\sreserve_byte\svalue\nis\s31\sor\sgreater.\s[forum:/forumpost/e807885dc5|forum\spost\se807885dc5].
- D 2021-01-21T21:36:25.588
 -C Merge\sfixes\sfrom\strunk.\s\sOmit\sa\sconditional\sin\s\nsqlite3IsLikeFunction()\sthat\swas\smade\sunreachable\sby\sthis\senhancement.
 -D 2021-01-22T20:28:30.345
++C Enhance\sthe\squery\splanner\sto\senable\sit\sto\suse\san\sindex\sfor\sIS\sNOT\sNULL\nconstraints,\seven\sif\sSTAT4\sis\snot\senabled.
++D 2021-01-22T21:23:12.101
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diff --cc manifest.uuid
index 949cc47910c69cd938b09bf25b9dacd4d185da87,35bccac4da8143c772db989f5253af2b69cebd9d..bdab433263d6acd410e196e5fe1a0f33e07ea61b
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--- 2/manifest.uuid
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