In the runtime there are specialized fast map routines for
    certain kep types. This CL lets the compiler make use of these
    functions, instead of always using the generic ones.
    
    As we now generate multiple versions of map delete calls, to make
    things easier we delay the expansion of the built-in delete
    function to flatten phase.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/180858

From-SVN: r271983