[llvm] [SPIRV] Add logic for OpGenericCastToPtrExplicit rewriting (PR #146596)

[Steven Perron via llvm-commits]

s-perron wrote:

> LGTM.
> 
> @Keenuts I'll let you give your thoughts about the change `getFlatAddressSpace`, but that matches expectations for the Kernel mode.

This works for shaders as well. 

https://github.com/llvm/llvm-project/pull/146596