[DependenceAnalysis] Check if result of getConstantPart is null

A seg-fault occurs due to a reference of a null pointer, which is
the value returned by getConstantPart. This function returns
null if the constant part is not found. The code that calls this
function needs to check for the null return value.

Differential Revision: http://reviews.llvm.org/D18718

llvm-svn: 265319