Support #![feature(unchecked_math)]

[antoyo]

The following code:

#![feature(unchecked_math)]

pub fn nop(num: i32) -> i32 {
    unsafe { (num.unchecked_mul(2)) / 2 }
}

gives:

example::nop:
        lea     eax, [rdi+rdi]
        sar     eax
        ret

while cg_llvm gives:

example::nop:
        mov     eax, edi
        ret

because it seems cg_gcc always enable checked maths.

[antoyo]

Done. Feel free to ask questions if you need help.

[antoyo]

This issue is actually much harder than I initially though, so I removed the good first issue tag.

It would be in this part of the code and would require changes to libgccjit.

You might want to pick another good first issue if you prefer.