Find a more elegant replacement for the codegen_simple_intrinsic macro

[danielsn]

Relevant comment in intrinsics.rs

        // Codegens a simple intrinsic: ie. one which maps directly to a matching goto construct
        // We need to use this macro form because of a known limitation in rust
        // `codegen_simple_intrinsic!(self.get_sqrt(), Type::float())` gives the error message:
        //   error[E0499]: cannot borrow `*self` as mutable more than once at a time
        //    --> src/librustc_codegen_llvm/gotoc/intrinsic.rs:76:63
        //    |
        // 76 |                 codegen_simple_intrinsic!(self.get_sqrt(), Type::double())
        //    |                 ---- ------------------------                 ^^^^ second mutable borrow occurs here
        //    |                 |    |
        //    |                 |    first borrow later used by call
        //    |                 first mutable borrow occurs here

        //  To solve this, we need to store the `self.get_sqrt()` into a temporary variable.
        //  Using the macro form allows us to keep the call as a oneliner, while still making rust happy.

[adpaco-aws]

Came across this while working on #727

@model-checking/rmc-devs do we still want to replace this macro?

I think the approach is not so bad and it allows us to handle all "builtin" intrinsics with a one-line call to this macro:

 "fmaf32" => codegen_simple_intrinsic!(Fmaf),

But maybe I'm missing something. Thoughts?

[tedinski]

My general thought is it'd be nice not to use macros at all in this code, but this is a low priority, there are 13 of them in there, and I don't think this one in particular is a special problem as far as I can tell.