PR14904: Segmentation fault running pass 'Recognize loop idioms'

The root cause is mistakenly taking for granted that 
    "dyn_cast<Instruction>(a-Value)"
return a non-NULL instruction.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172145 91177308-0d34-0410-b5e6-96231b3b80d8
fix comments a bit

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172146 91177308-0d34-0410-b5e6-96231b3b80d8