[analyzer] Merge implementations of SymInt, IntSym, and SymSym exprs

Summary:
SymIntExpr, IntSymExpr, and SymSymExpr share a big portion of logic
that used to be duplicated across all three classes.  New
implementation also adds an easy way of introducing another type of
operands into the mix.

Differential Revision: https://reviews.llvm.org/D79156