[LegalizeTypes] Remove WidenVecRes_Shift and just use WidenVecRes_Binary

This function seems to allow for the shift amount to have a different type than the result, but I don't think we do that anywhere else for vector shifts. We also don't have any support for legalizing the shift amount alone if the result is legal and the shift amount type isn't. The code coverage report here shows this code as uncovered http://lab.llvm.org:8080/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp.html

Differential Revision: https://reviews.llvm.org/D86475