[LegalizeTypes] Remove unnecessary if around switch in ScalarizeVectorOperand...

[LegalizeTypes] Remove unnecessary if around switch in ScalarizeVectorOperand and SplitVectorOperand. NFC

The if was checking !Res.getNode() but that's always true since
Res was initialized to SDValue() and not touched before the if.

This appears to be a leftover from a previous implementation of
Custom legalization where Res was updated instead of returning
immediately.