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

[Justin Cai via llvm-commits]

jzc wrote:

```
warning: failed to infer data layout: unable to get target for 'spirv64-unknown-unknown', see --version and --triple.
WARNING: failed to create target machine for 'spirv64-unknown-unknown': unable to get target for 'spirv64-unknown-unknown', see --version and --triple.
```
fix in #147977


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