Fix i256::MAX

[TDecking]

No description provided.

[tgross35]

Good catch, thanks. Would you mind updating the version at

compiler-builtins/libm/src/math/support/big.rs

Lines 84 to 87 in a4c748f

	const MAX: Self = Self {
	lo: u128::MAX,
	hi: u128::MAX << 1,
	};

as well? (I'll hopefully be deduplicating these to share the libm version in the near future since it's a bit faster).

I should figure out a way to sanity check our constants somehow (maybe also against rug like libm-test/tests/u256.rs).

[TDecking]

@tgross35 Done.

[tgross35]

Thanks!