CodeGen: Full support for isl_pw expressions in modified access functions.

This also adds support for modifiable write accesses (until now only read
accesses where supported). We currently do not derive an exact type for the
expression, but assume that i64 is good enough. This will be improved in future
patches.

Contributed by: Yabin Hu <yabin.hwu@gmail.com>

llvm-svn: 153319