[SDAG] avoid crash from mismatched types in scalar-to-vector fold

This bug was introduced with D136713 / 54eeadcf .

As an enhancement, we could cast operands to the expected type,
but we need to make sure that is done correctly (zext vs. sext).
It's also possible (but seems unlikely) that an operand can have
a type larger than the result type.

Fixes #58661