[clang] [CIR][AMDGPU] Use intrinsic type for dpp signature detection (PR #226921)

[Steffen Larsen via cfe-commits]

steffenlarsen wrote:

> > Note that this is not an RFC as it changes the signedness in parts of the builtin signatures.
> 
> Did you mean NFC?

I most definitely did! The description has been amended.

https://github.com/llvm/llvm-project/pull/226921