[mlir][GPU] add required address space cast when lowering to LLVM

The runtime functions `memset` and `memcpy` are lowered are declared with pointers to the default address space (0) while their ops however are compatible with memrefs taking any address space.
Such cases do not cause any issues with MLIRs LLVM Dialect due to `bitcast`s verifier being too lenient at the moment, but actual LLVM IR does not allow casting between address spaces using `bitcast`: https://godbolt.org/z/3a1z97rc9

This patch fixes the issue by inserting an address space cast before the bitcast, to first cast the pointer into the correct address space before doing the bitcast.

Differential Revision: https://reviews.llvm.org/D143866