Fix failure reported by Sebastian of test/Analysis/ptr-arith.c when the target

is 64-bit. I used his suggestion of doing a direct bitwidth/signedness
conversion of the 'offset' instead of just changing the sign. For more
information, see:

http://lists.cs.uiuc.edu/pipermail/cfe-dev/2009-March/004587.html

llvm-svn: 66892