Additional steps to help ensure that scalar subqueries are only evaluated

once even if they are used in multiple places within the query.  This fixes
a performance regression reported on the mailing list and caused by
check-in [531eca6104e41e43] which was a fix for ticket
[787fa716be3a7f650c]. Think of this check-in as an improved fix for
that ticket.

FossilOrigin-Name: e130319317e761193890ef75787a3c114b61166f518df019697be8e2c820ec54

diff --cc manifest
index c50fdb0addf0afead2a3825957b921baa9e89d91,a2a2d2d0023823da0b169aab2e522f95030f05c6..a66d29db97fd68e094e5b5039a9020d391fda760
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Small\schanges\sto\sthe\sOP_OpenEphemeral\sopcode\sto\simprove\stestability.
- D 2018-12-31T17:58:05.349
 -C Remove\san\soptimization\sthat\scan\sno\slonger\soccur,\sbeing\ssuperceded\sby\sthe\nsubquery-reuse\soptimization.\s\sPut\san\sassert\sin\splace\sof\sthe\soptimization\nto\sdetect\sif\sthe\sneed\sfor\sthis\soptimization\sever\sreturns.
 -D 2018-12-31T20:13:12.301
++C Additional\ssteps\sto\shelp\sensure\sthat\sscalar\ssubqueries\sare\sonly\sevaluated\nonce\seven\sif\sthey\sare\sused\sin\smultiple\splaces\swithin\sthe\squery.\s\sThis\sfixes\na\sperformance\sregression\sreported\son\sthe\smailing\slist\sand\scaused\sby\ncheck-in\s[531eca6104e41e43]\swhich\swas\sa\sfix\sfor\sticket\n[787fa716be3a7f650c].\sThink\sof\sthis\scheck-in\sas\san\simproved\sfix\sfor\nthat\sticket.
++D 2018-12-31T20:39:37.863
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  F Makefile.in d8b254f8bb81bab43c340d70d17dc3babab40fcc8a348c8255881f780a45fee6
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++Z 5fcc6f0e6119eb4a72b2bef9abdccc87

diff --cc manifest.uuid
index 2ecc2fe117360fde0104622885081d8d5f7df825,ae415af0abc076cdbb3ef74c2713c70a386c02be..53c7e8c45e7251e7fa9400ac8d09268579f13ccb
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
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++e130319317e761193890ef75787a3c114b61166f518df019697be8e2c820ec54