[NFC] Clean up Literal copy constructor

[kripken]

Diff without whitespace is smaller.

• HeapType::ext was handled in two places. The second place was wrong, but not reached.
• Near the end all we have left are refs, so no need to check isRef etc.
• Simplify the code to get the heap type once.

[tlively]

If we really wanted big simplicity wins here, we could use std::variant and get rid of all our custom copy constructors, destructors, etc.

[kripken]

Agreed that a larger cleanup to std::variant might make sense eventually. Literal is some of the oldest/most basic code in Binaryen, from long before we supported C++17 or C++17 even existed...