TableGen/AsmMatcherEmitter: Bring sorting check back under EXPENSIVE_CHECKS

Bring the sorting check back that I removed in r288655 but put it under
EXPENSIVE_CHECKS this time. Also document that this the check isn't
purely about having a sorted list but also about operator < having the
correct transitive behavior.

Apply the same to the other check in the file.

llvm-svn: 288693