[llvm] [SPIRV] Enable DCE in instruction selection and update tests (PR #168428)

[Steven Perron via llvm-commits]

s-perron wrote:

@michalpaszkowski Can you take a look from the Kernel perspective? I ran the expensive checks, and it does not introduce any new errors. There will be some follow up to remove more dead code, but it require more investigation.

https://github.com/llvm/llvm-project/pull/168428