[DAGTypeLegalizer] Handle ZERO_EXTEND of promoted type in WidenVecRes_Convert.
On SystemZ, a ZERO_EXTEND of an i1 vector handled by WidenVecRes_Convert()
always ended up being scalarized, because the type action of the input is
promotion which was previously an unhandled case in this method.
This fixes https://bugs.llvm.org/show_bug.cgi?id=47132.
Differential Revision: https://reviews.llvm.org/D86268
Patch by Eli Friedman.
Review: Ulrich Weigand