constant folding fails for constant exponentiation

[charles-cooper]

Version Information

• vyper Version (output of vyper --version): v0.4.0rc2

What's your issue about?

repro:

N: constant(uint256) = 3
N2: constant(uint256) = N**N

results in:

vyper.exceptions.InvalidOperation: Cannot apply an overflow check on exponentiation when both the base and power are unknown at compile-time. To perform this operation without an overflow check, use `pow_mod256(N, N)`

this appears to be a regression from 0.3.10