Regalloc hints

[sarranz]

Description

Experimental hints for the register allocator requested by @bgregoir

Checklist

• Add a changelog entry in changes if the PR is a user-visible change
• Add one or several tests to compiler/tests if it makes sense, especially if it is a bug fix

[sarranz]

I removed the "conflict" annotation. One thing worth noting:

export fn main() -> reg u64 {
    #[force_regalloc=RCX] reg u64 x = 0;
    return x;
}

fails even if the compiler introduces renaming, because the renames inherit the annotations. I don't know if it's a feature or a bug.