Revert the rule that considers comparisons between two pointers in the

same object to be a non-capture; Duncan pointed out a way that such
a comparison could be a capture.

Make the rule that considers a comparison against null more specific,
and only consider noalias return values compared against null. This
still supports test/Transforms/GVN/nonescaping-malloc.ll, and is not
susceptible to the problem Duncan pointed out with noalias arguments.

llvm-svn: 89468