[Ada] Prepare for more efficient signed integer overflow checking

[Paolo Bonzini]

> Expansions for addition, subtraction and negation will no longer be
> widened, but instead operands are checked before performing the
> operation. Multiplication will still be widened if neither operand
> is constant and if the type is not a 64-bit type.

This is 90% of the work needed to reenable -ftrapv.  Great.

Paolo