ConstantFold: Fold getelementptr (i32, i32* null, i64 undef) to null.

Transforms/IndVarSimplify/2011-10-27-lftrnull will fail if this regresses.
Transforms/GVN/PRE/2011-06-01-NonLocalMemdepMiscompile.ll has been changed to still test what it was
trying to test.

llvm-svn: 302446