Debug Info Verifier: disable it by default.

Debug info verifier is part of the verifier which is a Function Pass.
Tot currently tries to pull all reachable debug info MDNodes in each function,
which is too time-consuming. The correct fix seems to be separating debug info
verification to its own module pass.

I will disable the debug info verifier until a correct fix is found.

For Bill's testing case, enabling debug info verifier increase compile
time from 11s to 11m.

llvm-svn: 194986