Introduced in rust-lang/rust#55627

This will reduce duplication between rustc_codegen_llvm and rustc_codegen_cranelift and will make it easier to implement not yet implemented stuff.

Blocked on rust-lang/rust#56048 (rustc_codegen_ssa didn't get included in the sysroot)

I am working on this in the wip_use_cg_ssa branch.