Move the dominator verification code out of special code embedded within

Move the dominator verification code out of special code embedded within
the PassManager code into a regular verifyAnalysis method.

Also, reorganize loop verification. Make the LoopPass infrastructure
call verifyLoop as needed instead of having LoopInfo::verifyAnalysis
check every loop in the function after each looop pass. Add a new
command-line argument, -verify-loop-info, to enable the expensive
full checking.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82952 91177308-0d34-0410-b5e6-96231b3b80d8