DAGCombiner: Avoid generating illegal vector INT_TO_FP nodes

DAGCombiner::reduceBuildVecConvertToConvertBuildVec() was making two
mistakes:

1. It was checking the legality of scalar INT_TO_FP nodes and then generating
vector nodes.

2. It was passing the result value type to
TargetLoweringInfo::getOperationAction() when it should have been
passing the value type of the first operand.