Use `LLVMIntrinsicGetDeclaration` to completely remove the hardcoded intrinsics list
Follow-up to rust-lang/rust#142259
This also needs a rustc-perf run, because `Intrinsic::getType` can be expensive
`@rustbot` label A-LLVM A-codegen T-compiler
r? `@workingjubilee`
cc `@nikic`