[LCG] Remove the use of the parent sets to compute connectivity when

[LCG] Remove the use of the parent sets to compute connectivity when
merging RefSCCs.

The logic to directly use the reference edges is simpler and not
substantially slower (despite the comments to the contrary) because this
is not actually an especially hot part of LCG in practice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310161 91177308-0d34-0410-b5e6-96231b3b80d8