[SelectionDAG] In PromoteIntRes_BITCAST, when the input is TypePromoteFloat,...

[SelectionDAG] In PromoteIntRes_BITCAST, when the input is TypePromoteFloat, make sure the output type is scalar. For vectors, use a store and load of temporary.

Previously if the result type was a vector, we emitted a FP_TO_FP16 with a vector result type which isn't valid.

This is basically the opposite case of the root cause of PR38533.

llvm-svn: 339535