Add the left join strength reduction optimization. Enhance the push-down

optimization so that it works with many LEFT JOINs.

FossilOrigin-Name: dd568c27b1d7656388ea5b4132cc0265aedd7348d265d8e8c7412b00b28a31aa

diff --cc manifest
index 8266e850125291e6e8933f49674f17a925356919,1e671d756ca258a92f414d6d6d251f75722b5792..fce3efb027de4bcb5c893b4a060dcb7805a872cb
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Add\sthe\s--valid-sql\soption\sto\sthe\soptfuzz\stest\sprogram.
- D 2018-03-22T11:28:31.684
 -C Relax\sLEFT-JOIN\srestrictions\son\sthe\spush-down\soptimization.
 -D 2018-03-21T01:59:46.055
++C Add\sthe\sleft\sjoin\sstrength\sreduction\soptimization.\s\sEnhance\sthe\spush-down\noptimization\sso\sthat\sit\sworks\swith\smany\sLEFT\sJOINs.
++D 2018-03-22T12:00:43.103
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 7016fc56c6b9bfe5daac4f34be8be38d8c0b5fab79ccbfb764d3b23bf1c6fff3
@@@ -1715,7 -1712,7 +1715,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
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  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
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++R 829b4bcd2ce5d178bebfa1657a2d3cee
++T +closed b5d3dd8cb0b1e42ed0671a12d22af05194ea9522e4f41fd4bb0deff70b8b0757
  U drh
- Z 865cfc6e4e0c84a2365c19e9bd9ce995
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++Z f7b572645a2b0a1f5c79c87cac8e08a9

diff --cc manifest.uuid
index 2d0756f5dd2e160292c61799315ab2aa7c055331,edbd073aded90b2e791400d54043350cd0d9c66f..e31112db169959570ed83ca4736fc07efb62dda2
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- a8dfeec73b069f2dd7371c2792b36f152945d32120843db5361ff7e04de177bc
 -b5d3dd8cb0b1e42ed0671a12d22af05194ea9522e4f41fd4bb0deff70b8b0757
++dd568c27b1d7656388ea5b4132cc0265aedd7348d265d8e8c7412b00b28a31aa