Rework logical and/or type rules on contextual bool

[dcodeIO]

An attempt to fix #1136 by refining the existing special case for contextual bool on logical LHS && RHS and LHS || RHS.

For a bit of background: We can't model this exactly like JS because it would involve a union typeof(LHS) | typeof(RHS), so the compiler currently compiles LHS with the outer contextual type allowing it to deviate and RHS with LHS's type as the contextual type forcing an implicit conversion to get a single AOT-compatible type.

The important difference this PR introduces is that the compiler no longer attempts to implicitly convert the RHS expression to the LHS type if the outer contextual type is bool or void, instead allowing both LHS and RHS to have different types, which is fine since subsequent codegen converts each to a bool anyway.

Allows us to write

if (someObject && someInt)
if (someObject && someBool)
if (someObject || someInt)
if (someObject || someBool)
...

but also has the (unfortunate?) side-effect that the following magically works in AS where TS would complain:

function test(): bool {
  return someObject || someInt;
}