Reorder Type fields to make various isa< > check more concise

Depending on the order of fields, some isa < > checks can be faster because of
tests that check a range of type, leading to assembly simplification.

To find a relevant ordering, I... brute-forced the permutation among the derived
types and pick the combination that resulted in the smallest libLLVM-11.so.

On my laptop (x86_64), this reduces the size of libLLVM-11.so from 127344064 bytes to 127335336,
that's 8728 bytes shaved without much effort.

Also removed obsolete comments in the process.

Differential Revision: https://reviews.llvm.org/D79996