Tighten Int.to bounds and add twos-complement bitcount

[tiehuis]

Tightens the to function bounds and adds a twos-complement bit count variant which is useful for converting from comptime_int to a fixed size in the self-hosted compiler.

I've renamed bitcount as well to be clearer about what it is actually doing. I haven't explored the zero-sized integer case issues at all yet.

[andrewrk]

LGTM.

zig/src-self-hosted/ir.zig

Lines 1735 to 1736 in 58c5f94

	break :fits (from_int.positive or from_int.eqZero() or int.key.is_signed) and
	int.key.bit_count >= from_int.bitcount();

So I would change this to:

break :fits (from_int.positive or from_int.eqZero() or int.key.is_signed) and 
int.key.bit_count >= from_int.bitCountTwosComp(); 

Does that look right?

[tiehuis]

Think you'd want something more like this:

if (!int.key.is_signed and !from_int.positive) {
    break :fits false;
}

const req_bits = from_int.bitCountTwosComp() + @boolToInt(from_int.positive and int.key.is_signed);
break :fits int.key.bit_count >= req_bits

Signed never fits in an unsigned target and we need one extra bit for a positive source to a signed target.

[tiehuis]

With the special-cased zero fitting in any target:

if (from_int.eqZero()) {
    break :fits true;
}

[tiehuis]

It may make sense to add a fits function to Int itself for this purpose since this seems pretty general. It'd pretty much be like to except we would bother returning the converted value.