[clang][dataflow] Fix double visitation of nested logical operators

Sub-expressions that are logical operators are not spelled out
separately in basic blocks, so we need to manually visit them when we
encounter them. We do this in both the `TerminatorVisitor`
(conditionally) and the `TransferVisitor` (unconditionally), which can
cause cause an expression to be visited twice when the binary
operators are nested 2+ times.

This changes the visit in `TransferVisitor` to check if it has been
evaluated before trying to visit the sub-expression.

Differential Revision: https://reviews.llvm.org/D125821