Add maximum threshold for number of blocks per function #951

[nalmt]

To fix this case (#951) that may take forever to compile:

function %a(){
ebb477777777:
}

We decide to define a maximum threshold for the number of blocks in functions.

Based on a large WASM program (https://github.com/mozilla/perf-automation/blob/master/benchmarks/wasm-misc/AngryBots.wasm), its IR functions does not exceed 1414 blocks. A number 100 times greater (100,000 blocks) seems (currently) enough to define our maximum threshold.

To make this quick benchmark the cranelift-wasm/src/func_translator.rs file has been modified like this:

static mut MAX: usize = 0;

pub fn translate_from_reader<FE: FuncEnvironment + ?Sized>(...) {

    [...]

    builder.finalize();

    // the compiler is single threaded
    unsafe {
        if func.dfg.num_ebbs() > MAX {
            MAX = func.dfg.num_ebbs();
            println!("MAX {}", MAX);
        }
    }

    Ok(())
}