Don't always do optimistic HW intrinsic expansion (#89282)

Don't always do optimistic HW intrinsic expansion (#89282)

"Optimistic" expansion means we generate a method body for `IsSupported` that returns true/false depending on the currently running CPU. Don't do this when `--instruction-set:native` was specified because native should mean "exactly this". Also don't do this when optimizing for size (saves 0.6% on hello world). I was going back and forth whether to do this when user specifies instruction sets manually, but decided it would be a "breaking" change.