C#: Remove redundant conjunct in ssaDefReachesReadWithinBlock

[hvitved]

My attention was drawn to this predicate because of a bad join-order

[2021-07-15 12:06:35] (483s) Tuple counts for SsaImplCommon::SsaDefReaches::ssaDefReachesReadWithinBlock#2#ffff/4@fe48da:
                      5261258218 ~1%     {5} r1 = JOIN SsaImplCommon::SsaDefReaches::ssaDefReachesRank#2#ffff_0312#join_rhs WITH SsaImplSpecific::variableRead#2#cpe#123#fff_021#join_rhs ON FIRST 2 OUTPUT Lhs.0 'bb', Rhs.2 'i', Lhs.1 'v', Lhs.3, Lhs.2 'def'
                      631031     ~5%     {5} r2 = JOIN r1 WITH SsaImplCommon::SsaDefReaches::ssaRefRank#2#fffff_01243#join_rhs ON FIRST 4 OUTPUT Rhs.4, Lhs.0 'bb', Lhs.4 'def', Lhs.2 'v', Lhs.1 'i'
                      631031     ~0%     {4} r3 = JOIN r2 WITH construct<TSsaRefKind#2,0>@dom#SsaImplCommon::SsaDefReaches::SsaRead#2#0#f ON FIRST 1 OUTPUT Lhs.3 'v', Lhs.2 'def', Lhs.1 'bb', Lhs.4 'i'
                                         return r3

where ssaDefReachesRank and variableRead were joined on just bb and v. It turns out that the variableRead(bb, i, v, _) conjunct is actually not needed, since it is implied by ssaRefRank(bb, i, v, SsaRead()). Removing the conjunct therefore both simplifies the logic, and improves the join order:

[2021-07-15 12:20:59] (4s) Tuple counts for SsaImplCommon::SsaDefReaches::ssaDefReachesReadWithinBlock#2#ffff/4@d86644:
                      941503  ~0%     {4} r1 = SCAN SsaImplCommon::SsaDefReaches::ssaDefReachesRank#2#ffff OUTPUT In.0 'bb', In.3 'v', In.2, In.1 'def'
                      941503  ~1%     {5} r2 = JOIN r1 WITH SsaImplCommon::SsaDefReaches::ssaRefRank#2#fffff_02413#join_rhs ON FIRST 3 OUTPUT Rhs.4, Lhs.0 'bb', Lhs.3 'def', Lhs.1 'v', Rhs.3 'i'
                      631031  ~0%     {4} r3 = JOIN r2 WITH construct<TSsaRefKind#2,0>@dom#SsaImplCommon::SsaDefReaches::SsaRead#2#0#f ON FIRST 1 OUTPUT Lhs.3 'v', Lhs.2 'def', Lhs.1 'bb', Lhs.4 'i'
                                      return r3

https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/1203/