Analysis: Return early in isKnownNonNullAt for ConstantData

Analysis: Return early in isKnownNonNullAt for ConstantData

Check and return early for ConstantPointerNull and UndefValue
specifically in isKnownNonNullAt, and assert that ConstantData never
make it to isKnownNonNullFromDominatingCondition.

This confirms that isKnownNonNullFromDominatingCondition never walks
through the use-list of an instance of ConstantData.  Given that such
use-lists cross module boundaries, it never really made sense to do so,
and was potentially very expensive.

llvm-svn: 282333