adjust documented register constraints to match https://llvm.org/docs/LangRef.html#supported-constraint-code-list
adjust documented register constraints to match https://llvm.org/docs/LangRef.html#supported-constraint-code-list

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