Add support for complex short-circuiting in non-conditional flows

[sonalmahajan15]

NilAway can support prevalently found short-circuiting in non-conditional logical expressions. Examples of supported expressions are shown below (refer to nonconditional.go for detailed test cases).

return x != nil && x.f != nil && x.f.g != nil && x.f.g.h == 1

return x == nil || x.f == nil || x.f.g == nil || x.f.g.h == 1

Although not common, NilAway reports false negatives for complex short-circuit expressions, such as below.

return (a != nil || b == nil) && *a == 1

return !(a != nil && b != nil) && *a == 1

Following are the ideas that can be used to extend support for the above cases:

1. Short-circuit expressions in conditional flows (e.g., if (a != nil || b == nil) && *a == 1 {}) are well-supported in NilAway. So, the idea here is to transform the non-conditional expression to a conditional expression in the pre-process phase. However, this would require the AST to be modified. We currently don't do this since drivers such as nogo create a shared AST between all linters. However, if we deeply copy the AST in NilAway, then this idea could be a viable approach.
2. Emulate complier logic for evaluating short-circuit expressions.