Avoid ref when using format! in compiler

Clean up a few minor refs in `format!` macro, as it has a performance cost. Apparently the compiler is unable to inline `format!("{}", &variable)`, and does a run-time double-reference instead (format macro already does one level referencing).  Inlining format args prevents accidental `&` misuse.

Author: nyurik

src/abi/mod.rs

@@ -505,7 +505,7 @@ pub(crate) fn codegen_terminator_call<'tcx>(
                 let nop_inst = fx.bcx.ins().nop();
                 fx.add_comment(
                     nop_inst,
-                    format!("virtual call; self arg pass mode: {:?}", &fn_abi.args[0]),
+                    format!("virtual call; self arg pass mode: {:?}", fn_abi.args[0]),
                 );
             }