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.
fix comments a bit
[analyzer] Allow IvarInvalidation checker to suppress warnings via
assertions.

To ensure that custom assertions/conditional would also be supported,
just check if the ivar that needs to be invalidated or set to nil is
compared against 0.

Unfortunately, this will not work for code containing 'assert(IvarName)'