Fix CodeGen of pointer arithmetic in compound assignment statements.

 - Changed EmitSub to work on all cases (arithmetic, ptr - int, ptr -
   ptr) so that implementation matches that of other operators.

 - Modify EmitCompoundAssign to compute and perform the appropriate
   casts of left, right, and result types for the assorted pointer
   arithmetic cases.

 - Fix EmitSub (ptr - int) case to negate the rhs *after*
   extension. This is critical when the rhs is unsigned (and needs
   extension).

 - This fixes cfrac.

 - <rdr://6115726>

llvm-svn: 54392