Add support for all forms of getelementptr instruction

[dpaoliello]

Current support for the getelementptr instruction had two restrictions:

• No support for the vector-index form (https://llvm.org/docs/LangRef.html#vector-of-pointers).
• When indexing into a 0-sized array, if there were any subsequent indexes then the generated code would be incorrect (due to operator precedence).

This change removes these restrictions:

• Detect the vector-index form and generate a vector of pointers.
• Simplify and unify how indexes-after-the-first are generated to avoid the operator precedence issue.
• At each step of indexing, always take the address of the expression - this allows uniform chaining of indexing (we can always assume the receiver is a pointer) and no special casing for the outermost expression (as we need to return a pointer).
• Added tests for missing scenarios.