[DAGCombiner] When combining (sext_inreg (zext X), VT) -> (sext X) don't pass...

[DAGCombiner] When combining (sext_inreg (zext X), VT) -> (sext X) don't pass along the sext_inreg VT.

ISD::SIGN_EXTEND is only supposed to have one operand, but we
were creating it with 2 operands.

Since we basically never check for extra operands this went
unnoticed.