Folded Nodes Are Not Validated

[ritzdorf]

Version Information

• vyper Version (output of vyper --version OR linkable commit hash vyperlang/vyper@): b43ffac

Issue Description

When some node is folded the folded value is not validated. In the
general case, it is not an issue as the semantic validation phase will
catch the error. However, if the folded value is used in a type
annotation for a String or Byte type, the compiler will not raise.

POC

The following contract compiles although the size in byte of the String
is greater than the maximum value of uint256

x:String[max_value(uint256) + 10]