Add support for AllocaRegion extent with GDM.

One design problem that is emerging is the signed-ness problem during static
analysis. Many unsigned value have to be converted into signed value because
it partipates in operations with signed values. 

On the other hand, we cannot blindly make all values occuring in static analysis
signed, because we do have cases where unsignedness is required, for example,
integer overflow detection.

llvm-svn: 59957