Mark Reg::new_real and Reg::new_virtual as #[inline]

[bjorn3]

Otherwise they can't be inlined into users of regalloc.rs. On one profile it took 4 samples of the cranelift_codegen::isa::x64::abi::compute_arg_locs function, which has 12 samples. While this is of course not very accurate due to the very small sample count, inlining them should still give a small improvement.