[OpenCL] Fix bug in __builtin_astype causing invalid LLVM cast instructions

__builtin_astype is used to cast OpenCL opaque types to other types, as such, it needs to be able to handle casting from and to pointer types correctly.

Current it cannot handle 1) casting between pointers of different addr spaces 2) casting between pointer type and non-pointer types.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D25123

llvm-svn: 283114