[analyzer] Extend IdenticalExprChecker to check ternary operator results.

Warn if both result expressions of a ternary operator (? :) are the same.
Because only one of them will be executed, this warning will fire even if
the expressions have side effects.

Patch by Anders Rönnholm and Per Viberg!
[analyzer] Misc. tidying in IdenticalExprChecker.

Some things I missed when this first went in.