We do not support alias checks for base pointers defined inside the SCoP

The run-time alias check places code that involves the base pointer at the
beginning of the SCoP. This breaks if the base pointer is defined inside the
SCoP. Hence, we can only create a run-time alias check if we are sure the base
pointer is not an instruction defined inside the scop. If it is we refuse to
handle the SCoP.

This commit should unbreak most of our current LNT failures.

Differential Revision: http://reviews.llvm.org/D5483