[SelectionDAG] Widen vector results of SMULFIX/UMULFIX/SMULFIXSAT

Summary:
After the commits that changed x86 backend to widen vectors
instead of using promotion some of our downstream tests
started to fail. It was noticed that WidenVectorResult has
been missing support for SMULFIX/UMULFIX/SMULFIXSAT. This
patch adds the missing functionality.

Reviewers: craig.topper, RKSimon

Reviewed By: craig.topper

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66051

llvm-svn: 368540