rustc: combine Instance::fn_sig_noadjust and Instance::fn_sig.
rustc: combine Instance::fn_sig_noadjust and Instance::fn_sig.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust