Add equivalent to Rust's std::hint::blackbox

[TomAFrench]

In the short term (and complementary to debug mode in the future), we could add something similar to Rust's blackbox function. This would just be an identity function which is listed as having side-effects, which would block any SSA optimizations from being applied to the outputs.

Originally posted by @TomAFrench in #2128

One tool that I've found myself needing a few times when minimising bug reproductions is a way to block our SSA optimization passes being applied to a value. This would also allow us to prevent test programs from being optimized down to nothing and have them resemble runtime code more closely.